GB2413891A - A cold cathode fluorescent lamp with a uniform lighting profile - Google Patents

Abstract

A cold cathode fluorescent lamp (CCFL) comprising a linear tube 400 with an inner surface on which a luminescent substance is distributed, a longitudinal distribution 255 of the luminescent substance having a minimum at a first point on the inner surface and a luminescent substance density greater than the minimum at each of a second 450 and third points 451 on the inner surface, the first point longitudinally located between the second and third points 450, 451. This provides a lamp tube with a uniform lighting profile. A method of making a lamp tube 400 having a first end 410 and a second end 420 comprising introducing a first quantity of a luminescent substance into the first end 410 of the lamp tube 400 and introducing a second quantity of a luminescent substance into the second end 420 of the lamp tube 400 is also disclosed.

Description

LAMP TUBE IIAVING A UNIFORM LIGHTING PROFILE AND A

MANUFACTURING METHOD THEREF()R

TECHNICAL FIELD OF THE INVENTION

This invention relates to lamp tubes and, more particularly, to a lamp tube having a umfonn lighting profile and to a treatment process for producing same.

13_GROUND OF THE INVENTION Optical scanners generate machine-readable image data representative of a scanned object such as an image on a paper document or other media. Flatbed optical scanners are stationary devices which have a transparent platen upon which the media or object to be scanned is placed. Equipment such as flat bed scanners, film scanners, copters and some digital cameras may use a linear cold cathode fluorescent lamp (CCFL) as the light source. The media or object is scanned by scqucnbally imaging narrow strips or scan Ime portions of the media or object by an Imaging apparatus such as a charge-coupled device (CCD). The imaging apparatus produces image data which Is representative of each scan line portion of the scanned media or object. A linear arrangement of light sensitive elements, such as CCD photodetectors, is used to convert light into electric charges. There are many relatively low-priced color and black and white, one-dimensional array CCD photodetectors available for image scanning systems. Electronic imaging systems may alternatively use two-dimensional arrays of light sensitive elements such as CCD arrays. Ilowever, these arrays are expensive because they have low manufacturing yields. Linear photodetectors cost much less than array detectors because they are much smaller and have higher manufacturing yields.

While linear CCFLs are bright, inexpensive, and reliable, they also have one major disadvantage - they have a non-uniform illumination intensity profile that requires corrective analog or digital gain to normalize. These devices suffer from low sgnal-to-noise ratios at the ends of the scan lines due to decreased light intensity on the object or media and through the optical system.

SUMMARY OF THE INVENTION

In accordance with an embodiment of the present invention, a method of treating a lamp tube having a first end and a second end comprising introducing a first quantity of a luminescent substance mto the first end of the lamp tube and Introducing a second quantity of a luminescent substance into the second end of the lamp tube is provided.

In accordance with another embodiment of the present invention, an illumination source comprising a linear tube having a first end and a second end and an inner surface having a luminescent substance distributed thereon, a longitudinal dstubuton of the luminescent substance having a minimum at a first point of the mner surface and a luminescent substance density greater than the minimum at each of a second and third point of the inner surface, the first point longitudinally located between the second and third points, is provided.

F3RIEF DESCRlP l ION OF THE DRAWINGS For a more complete understanding of the present invention, the objects and advantages thereof, reference is now made to the following descriptions taken in connection with the accompanying drawings In which: I;ICURE I is a diagram representing an embodiment of a scan media document that may be scanned by an imaging system according to the present invention; FIGURE 2 Is a diagram illustrating Illumination of a scan object contributed from a single point of an illumination source; Fl(GURE 3 is a diagram illustrating the cumulative 11uminahon of a midpoint of a scan object resulting from the entirety of the Illumination source; FIGURE 4 is a diagram illustrating the cumulative illumination of an endpoint of a scan object resulting from the entirety of the illumination source; I;lGUREs 5A-5B, respectively, illustrate a radiation profile and a lighting profile of an illumination source having a uniform luminescent substance distribution and a radiation profile and a lighting profile of an illumination source having a typical luminescent substance distribution as is known in the prior art; FlGUREs 6A-6D illustrate an embodiment of an illumination source according to the present invention, and exemplary luminescent substance density profiles resulting therctrom; FIGURE 7 is a diagram Illustrating a radiation profile and hghtng profile of an magmg system according to the teachings of the present invention utilizing the Illumination source described with reference to FIGURE 6; and FlGUREs SA- 8J illustrate cross-sectional News of a lamp tube undergoing a treatment process for manufacturing the lamp tube with a non-linear luminescent distribution all according to an embodiment of the Invention.

DF.'rAlLeD DESCRIPTION OF THE DRAWINGS

I'he preferred embodiment of the present invention and its advantages are best understood by referring to FIGURES I through 8 of the drawings, like numerals being used t'or hke and corresponding parts of the various drawings.

In FIGURE 1, there is illustrated a scan media, such as for example and not by way of limitation, a media 100 that may be scanned by an imaging system, for example a natbed scanner, digital camera, copier, film scanner, or another device.

'l'he magma system uses an illumination source, for example a hnear cold cathode fluorescent lamp (CCFL) having phosphor, or another luminescent substance, excited by mercury molecules or another ultra-violet radiation source, to seen sequential scan line portions 10A-ION of media 100. Other types of lamps are commonly used in imaging devices, such as xenon lamps having phosphors excited by ultra-violet radiation from xenon molecules in the lamp tube. A scan line is illuminated with a CCFL with a plurality of focal points on each scan line. The totality of the light striking a particular focal point can be considered to originate from a finite number of point sources along the CCFL. The light that comes into focus on a focal point is generally passed through an image forming system, t'or example an image stabilizer, a filter, an optic system, a single tens, a holographic lens or another device. The light is then passed to a photodetector where it Is converted to an electric charge. Generally, a plurality of electric charges are generated according to this technique for a given scan line. Once electric charges for a particular scan line have been produced, the charges for the next scan hne are generated. This general procedure is repeated until all scan hnes of media 100 have been imaged.

In FIGURE 2, there Is Illustrated an 11uminaton source, for example a CCFL 150, radiating light onto a scan object 160. Scan object 160 is representative of a scan line, for example seen line lOA, of scan media 100. In actuality, CCFL 150 radiates hght along a continuous, cylindrical source having collinear endpoints (the terminating ends of CCFL 150). For simplification of discussion, the light radiating from CC'FL 150 is considered to originate from a linear source comprised of a finite plurality of pomt sources 1 50A-150K colinearly located on CCFL 150.

Light rays are emitted froin each point source 150A-150K of CCFL 150 in multi-directons, for example light rays 150Fa-150Fk are emitted from point source 150F. Each pomt source 150A-lSOK emits light rays that impinge along scan object 160. Each pomt source, for example point source 150F, radiates a plurality of light rays that impinge at various points 160a-160k along scan object 160. The intensity of illumination of any given point 160a- 160k is a function of the distance between the point 160a-160k and the point source 150A-1SOK contributing to the illumination of the pomt 160a- 160k. Specifically, the intensity of illumination provided by a given point source 1 50A- 150K is proportional to 1/r2, where r = d(cos(cr))', d is the distance between the illuminated point 160a-160k and the illuminating point source, and a Is an angle of impingement of the light rays originating from point sources 150A-150K with a particular point 160a-160k. Thus, the cumulative, or total, Illumination intensity is an integral quantity inversely proportional to the square of r. Thus, point 160f will have a greater illumination intensity resulting from point source 150F than the illumination intensity of any other points 160a-160e and 160g-160k due to the direct, that is perpendicular, impingement of light ray 150Ff with point 160f. The illumination intensity for all other points 160a-160e and 160g-160k resulting from light radiated from point source 150F Will decrease with an increase In the distance therebetween.

The cumulative illumination of point 1 60f of scan object 160 can be considered to be an integral of the light radiating from along the entirety of point sources 150A-150K. As illustrated in FIGURE 3, the total illumination intensity of point 160f of scan object 160 is an integral of the Summation contabutons from various light rays lSOA150Kr originating from along the Icagth of CCFL 150. The collection of hght rays 150A-150Kf can be considered to include a principal hght ray 150T;, Impinging on point 160f pcrpendcularly therewith, that Is principal light ray 1 50Fr impinges point 160f at an impingement angle a of zero, while remaining light rays 15()Af-150Fif and 150Gf-150Kf mpingc point 160f at various angles of impingement a greater than zero. As mentioned above, a fight ray's contribution to the illumination intensity of point 160f decreases with an increase in the distance between the Illumination source and the illuminated point 160a-160k. Thus, light ray 150Af provides less radiation to point 1 60f than, for example, light ray 1 50Rf.

If CCI;L 150 were an idealized (that Is radiating light rays along the length thereof with uniform intensity) and Infinitely long light source, each point 160a-160f would be illuminated with identical intensity. However, because CCFL 150 is finite m length, a non-uniform illumination intensity profile is exhibited along scan object 1 5 160 that results in less intense illumination at points near the cud of scan object 160.

As Illustrated in FIGURE 4, the light radiating on point 160k at a far end of scan object 160 has a principle ray 150Kk having auxiliary rays 150Ak-150Jk originating from only one side of principle ray 150Kk. Thus, the illumination intensity of point 160k will be less than the illumination intensity of, for example, point 160f because the illumination of point 160k is, in effect, an integral of point source illuminations over nearly 90 degrees while the 11uminaton of point 160f is an integral of point source illuminations over nearly 180 degrees. The result Is a non-uniform illumination mtensty profile 210 as shown in FIGURE 5A. Radiation profile 200 illustrates an approximate radiation profile along the length of the illumination source, for example CCFL 150, having a uniform distribution of a luminescent substance along the surface of CCFL 150. For example, a typical CCFL comprises a scaled glass tube with a luminescent substance, such as phosphor, distributed along the inner surface thereof A CCFL having a surface with a uniform distribution of a luminescent substance will radiate fight of am form intensity along the length thereof, as illustrated by radiation profile 200. Notably, the radiation profiec 200 is a non integral measurement, that is each point of the radiation profile plot only indicates the Intensity of radiation from points (O through L) along the length of CCFL 150 whereas the illumination intensity profile 210 shows the Integral effect of Illumination at points 160a-160k of an object being illuminated by an illumination source havmg radiation profile 200. Pomts along a midsection of seen object 160 have a greater illumination than points near either of the endpoints, for example points 160a and 160k, of scan object 160 due to the aforedescribed integral effect of Rumination.

The non-uniform Illumination intensity profile 210 of the CCFL 150 may also have a secondary cause resulting from a well documented function of the light gathering capability of a typical lens used in image capturing systems. The contributory effect to the non-uniform illumination intensity profile 210 due to the light gathering eapabhties of a lens has been shown to be a cos4 function between the optical path centerline and a line drawn to the relevant area of the image. The overall effect causes an exponential loss of light as the angle increases at the endpoints of the seen object 100. Thus, imagmg systems such as scanners that utilize CCFLs suffer from low signal-to-noise ratios at the ends of the scan lines due to decreased light on the scan object, or page, and through the remaining optical system.

The non-umform illumination intensity profile 210 shown in FIGURE 5A results from CCFL 150 having a uniform phosphor, or other illumination substance, coating along the length of CCFL 150, as indicated by a Illumination substance density profile 195. However, the phosphor coating is often non-uniform along the length of a CCFL due to non-ideal properties of typical manufacturing techniques.

For example, a common manufacturing technique results in a uniform distribution of a luminescent substance around the circumference of the illumination source but also results in a non-uniform distribution of the luminescent substance along the longitudinal axis of the illumination source. In FIGURE 5B, there Is illustrated a typical CCFL 220 having a non-uniform distribution of an illumination substance on an Inner surface thereof as indicated by an illumination substance density profile 225.

A section (illustratively denoted by shaded area 220A) of CCFL 220 has a greater illumination substance density than the remammg portion of CCFL 220.

Consequently, the end of CCFL 220 having the greater Illumination substance density results in an Increased light intensity radiated from that end as illustrated by a skewed region 230A of radiation profile 230. The skewed region 230A results m a eounter effect that offsets the typical loss of illumination near the ends of a scan object due to the described integral effect of llummaton. A resulting Illumination intensity profile 240 has a more linear plot at the corresponding end and results in a reduction, or elimination, of the required corrective normalization at that end. The present invention advantageously exploits this phenomena. A novel lamp tube treatment process produces a lamp tube having a nonuniform illumination substance distribution that includes a luminescent substance density that is greater at both ends, rather than at a single end, of the tube than at a midsection of the tube - such a tube operable to provide an improved, uniform Illumination intensity profile.

In FIGURE 6A, there is illustrated a CCFL 250, or other illumination source, with a novel phosphor, or other luminescent substance, density distribution along the length thereof constructed according to the teachings of the present mventon. A midsection 260B of CCFL 250 has a generally constant phosphor density distribution as illustrated by luminescent substance density profile 255 (FIGURE 6B). The ends 260A and 260A2 of CCFL 250 have a higher phosphor density distribution compared to midsection 260B. While the Illustration shows CCFL 250 having areas of two different phosphor densities, it should be understood that ends 260A and 260A2 may have a non-constant phosphor density as well. For example, ends 260A and 260A2 may have a phosphor density distribution that increases toward the ends of CCFL 250 as illustrated by luminescent substance density profile 260 (FIGURE 6C). In fact, midsection 260B may also have a slightly increasing phosphor density distribution from its midpoint (point Ml) outward towards sections 260A and 260A2 as illustrated by the luminescent substance density profile 265 (FIGURE 6D). Thus, CCFL 250 is characterized most generally as having an Increasing phosphor density distribution outwardly from a midpoint Ml of CCFL 250 and has a corresponding minimum radiation intensity at the midpoint Ml of CCFL 250. The minimum radiation intensity may be commonly radiated from a portion of CCFL 250 including midpoint Ml and spanning outwardly therefrom towards either (or both) endpoint (O or L) to a point where the radiation intensity increases. The luminescent substance density distribution preferably provides a uniform Illumination intensity profile 310, as Illustrated in FIGURE 7, that results from a non-uniform radiation profile 300. As shown, illumination Intensity profile 310 is of approximately equivalent intensity at all points spanning the length of the seen object.

According to the present invention, to achieve uniform illumination mtensty profile 310, CCFL 250 preferably provides a non-unform radiation intensity along the length of CCFL 25O, that is the radiation profile 300 is preferably non-unifonn to compensate for the integral effects of illumination and/or lens losses as discussed hereinabove. As described with reference to FIGURE 6, a non-linear phosphor distribution Is used for obtamng an llummation intensity greater near ends 260A and 260A2 than along the midsection of CCFL 250. Preferably, the phosphor distribution of CCFL 250 is implemented such that radiation profiec 300 is the inverse of illumination intensity profile 210 Illustrated In FIGURE 5. Illumination with such a light source produces uniform illummaton of a scan object by compensating illumination at the ends of a scan object by Impinging principle rays thereon that are of greater intensity than principle rays radiated along the midseclon of the scan object.

FlGUREs 8A-8J, illustrate cross-sectional views of a lamp tube 400 at various stages of a treatment process that results In lamp tube 400 having a non-linear luminescent substance density distribution according to the teachings of the invention.

In a first step (FIGURE 8A), a lamp tube 400 is loaded into a luminescent substance coating machine. A luminescent substance, such as a phosphor solution, is next introduced Into first end 410 of tube 400 (FIGURE 8B). Dry air is then introduced into tube 400, for example at a second end 420 of tube 400, to dry the luminescent substance (FIGURE 8C). When the luminescent substance is dried, the luminescent substance density distribution generally appears as depicted in FIGURE 8D (shaded areas illustratively denoting areas of greater luminescent substance density than non shaded areas) and includes an area 450 having a high density of the luminescent substance.

To minimize the footprint area of the coating machine, typical manufacturing processes coat luminescent lamp tubes with lamp tube 400 vertically oriented although lamp tube 400 may be positioned at an acute angle as well. In doing so, the luminescent material is often pulled into the tube from a luminescent source located at the bottom (B) or first end 420 of tube 400. For manufacturing simplicity, the drying air is most often injected Into second end 420 of tube 400 opposing first end 410, that is the drying air is generally injected into the top (T) end of tube 400. The effect of such a process generally results In a uniform luminescent coating around the circumference of tube 400 but produces a difference In the end-to-end luminescent substance density distribution, that is a non-uniform luminescent substance density dstnbution along the longitudinal axis of the tube 400. This effect can be seen in FIGURE 8D where an area 450 proximate first end 410 has a greater luminescent substance density than the remaining portion of tube 400. The region 450 along tube 400 having a greater luminescent substance density does not generally have a sharp transition but rather Is a gradual change in luminescent substance density.

The present invention advantageously exploits the effect of producing a non uniform distribution of the luminescent substance at the bottom end of tube 400 when treating a tube by reversing the tube (FIGURE 8F) orientation within the tube treatment machine and repeating the general procedure described above. After ends 410 and 420 of the tube are reversed (such that end 410 occupies the position originally had by end 420, and vice versa), a predetermined quantity of the luminescent substance, for example a phosphor solution, is next introduced into second, or bottom, end 420 of tube 400 (FIGURE 8G) . Air is then introduced into tube 400 to dry the luminescent substance (FIGURE 8H), for example by injecting, or blowing, dry air into first end 410 (now located at the top (T) position in the treatment machine) of tube 400. The longitudinal distribution of the luminescent substance within tube 400 appears as generally illustrated m FIGURE 8I after the luminescent substance has dried. As illustrated, the entry of a second quantity of the luminescent substance and drying thereof in tube 400 after reversing the orientation results in a second area 451 having a high density of the luminescent substance in the end opposite first area 450. A portion 460 of first end 410 of tube 400 may next be cleaned for an internal electrode mount (FIGURE BE). Alternative electrode mounts Include external electrode mounts and combination internal and external electrode mounts. A portion 461 of second area 451 may then be cleaned for providing an electrode mount area. Accordingly, tube 400 has areas 450 and 451 proximate ends 410 and 420 that have higher surface densities of luminescent substance than that of a midsection 455 of tube 400.

It may be seen from the foregoing that an illumination source, such as a CCFL tube, having a non-uniform luminescent substance distribution may be produced according to the teachings herein. The illumination source generally includes areas of higher luminescent substance density near the ends of the illumination source. Higher intensity light is thereby radiated from the areas of high luminescent substance density when the tube is used in a lamp for illuminating an object so that a umform Summation intensity profile may be achieved.

Claims (10)

1. An illumination source comprising a linear tube (400) comprising a first end (410) and a second end (420), the tube having an inner surface having a luminescent substance distributed thereon, a longitudinal distribution density (255) of the luminescent substance having a minimum at a first point of the inner surface, the tube (400) having a luminescent substance density greater than the minimum at each ot a second (45()) and third point (451) of the inner surface, the first point longitudinally located between the second and third points (450; 451).

2. The illumination source according to claim 1, wherein a luminescent substance density of the second and third points (450; 451) are equivalent.

3. The illumination source according to claims 1 or 2, wherein the tube (400) includes a first electrode mount area (460) and a second electrode mount area (461), the second point (450) longitudinally located between the first point and the first electrode mount area (460), the third point (451) longitudinally located between the second point (450) and the second electrode mount area (461).

4. A method of making a lamp tube (400) having a first end (410) and a second end (420), the method comprising: introducing a first quantity of a luminescent substance into the first end (410) of the lamp tube (400); and introducing a second quantity of a luminescent substance into the second end (420) of the tube (400).

5. The method according to claim 4, wherein introducing a first quantity of a luminescent substance into the first end (410) of the lamp tube (400) further comprises positioning the first end (410) of the lamp tube (400) at a first location in a tube treatment assembly prior to introducing the first quantity ot the luminescent substance into the first end (410) of the lamp tube (400), the method further comprising the step of repositioning the tube (400) such that the second end (420) is positioned at the first location prior to introducing the second quantity of luminescent substance into the second end (420) of the tube (400).

6. 'I'he method according to claim 5, wherein positioning the first end (410) of the lamp tube (400) at the first location further comprises vertically orienting the tube (400) in the tube treatment assembly, and repositioning the tube (400) further comprises repositioning the tube (400) in a vertically oriented position.

7. The method according to any one of claims 4 to 6, wherein introducing a second quantity of a luminescent substance further comprises introducing a second quantity equivalent to the first quantity of a luminescent substance.

8. 'I'he method according to any one of claims 4 to 7, wherein introducing a first quantity of luminescent substance further comprises applying a vacuum to the second end (420) of the tube (400), the vacuum drawing the luminescent substance into the tube (400), and introducing a second quantity of luminescent substance further comprises applying a vacuum to the first end (410) of the tube (400), the vacuum drawing the luminescent substance into the tube (400).

9. The method according to any one of claims 4 to 8, wherein introducing a first quantity of a luminescent substance into the first end (410) of the lamp tube (400) further comprises introducing the first quantity of the luminescent substance into the first end (410) of a cold cathode fluorescent lamp tube (400), and introducing a second quantity of a luminescent substance into the second end (420) of the tube (400) further comprises introducing a second quantity of the luminescent substance into the second end (420) of the cold cathode fluorescent lamp tube (400).

10. The method according to any one of claims 4 to 9, wherein introducing a first quantity of a luminescent substance into the first end (410) of the lamp tube (400) further comprises introducing the first quantity of the luminescent substance into the first end (410) of a linear, cylindrical tube (400), and introducing a second quantity of a luminescent substance into the second end (420) of the tube (400) further comprises introducing a second quantity of the luminescent substance into the second end (420) of the linear, cylindrical tube (400).