JPH0671301B2 - Image reader - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION -Industrial field- The present invention relates to image reading in which an image is read by scanning a laser beam on a medium on which an image is recorded, such as an X-ray film, and detecting photoelectric conversion of the transmitted light. Regarding the device.

-Background of the invention-As is well known, in the medical field, a diagnostic system for diagnosing a disease by computer processing an image read from an X-ray film has become widespread. FIG. 12 shows a basic configuration example of an image reading apparatus for this purpose. The laser light emitted from the laser device 1 is converted into almost parallel light by the beam shaping lens 2 and is deflected by the polygon (rotating polygon mirror) 3.
To be scanned. Here, the scanned laser light is focused by the imaging lens 4, and scans the transmission image medium 5 in the arrow A direction (main scanning direction). The film 5 serving as a transparent image medium is conveyed by the conveying roller 6 in the direction of arrow B (sub-scanning direction). Then, the light intensity of the transmitted light of the film 5 is modulated by the density of the image, and the modulated transmitted light is detected by the light detection unit 7, and an electric signal carrying image information is obtained. 8 is a photodetector for obtaining a synchronization signal, 9
Is an amplifier for amplifying the image signal, and 10 is a signal shaping circuit for shaping the waveform of the detection signal from the photodetector 8.

-Problems to be Solved by the Invention- Fig. 13 shows the structure of the X-ray film 5 described above, and the support 11
The emulsion layer 12 is coated on both sides so as to sandwich it, and the protective layer 13 is further coated on the outer side thereof.

In beam scanning of such a film, reflection occurs due to a change in the refractive index at the interface between the film layers. In particular, when the light enters the protective layer from the air or from the protective layer to the air, the change in the refractive index is large, and there is usually about 4 to 5% of reflected light. Therefore, the 14th
As shown in the figure, it is divided into a component L ₁ that directly transmits incident light L ₀ and a component L ₂ that transmits the film after being reflected by B and C. There is an optical path difference δ between L ₁ and L ₂ .

δ = (BC + CD) n−BE = 2nh · cosφ ′ where n is the refractive index and h is the thickness.

When this optical path difference δ is an integral multiple of the wavelength of the reading laser beam, the two are most strengthened, and when they are deviated by half a wavelength, they are most weakened. The optical path difference δ changes depending on the scanning angle φ and the film thickness h, and as a result, the amount of transmitted light differs depending on the position of the film due to the combined optical path difference between the two, resulting in interference fringes. FIG. 15 shows the change in the phase difference depending on the scanning angle for a film of h = 175 μm, λ = 0.6328 μm, n = 1.52 as an example. The horizontal axis of the figure shows the scanning angle, and the vertical axis shows the phase difference δ normalized by the wavelength λ. That is, at the intersection with the horizontal line on the graph, the light intensities are strengthened and weakened between them, and as a result, interference fringes appear wider in the central portion of the scanning width than in the graph and denser toward both ends.

Further, although it is not possible to unconditionally specify the change in film thickness, interference fringes are observed as contour lines for each film thickness. If there is no change in the film thickness, this effect need not be taken into consideration, but it is practically impossible to reduce this thickness change to the wavelength order or less, and even if it is possible, it will be extremely costly. Invites up.

Further, if the reflectance of the film surface is lowered, the influence of such interference fringes can be prevented, but it is not desirable to attach an antireflection film to the read image film because it causes an extremely high cost.

SUMMARY OF THE INVENTION An object of the present invention is to provide an image reading apparatus which eliminates the occurrence of interference fringes due to its coherence when reading a film image with a laser beam.

-Means for Solving the Problems-In order to achieve the above-mentioned object, the present invention scans a surface of a transparent medium on which an image is recorded with a laser beam having a plurality of wavelengths to form a boundary surface between the front and back surfaces of the medium. In an image reading apparatus for reading the image from the transmitted light refracted by the method, when the refractive index of the medium is n and the thickness of the medium is h, the laser beam having a plurality of wavelengths λ ₁ , λ ₂ Each wavelength difference (Here, q = 0,1,2 ...) is proposed.

-Operation-By transmitting the medium with a laser beam having a plurality of wavelengths, the phase difference between the laser beam directly transmitted through the film image and the laser beam refracted twice at the front and back boundary surfaces of the film is different. The generation of the interference fringes is suppressed by making them different from each other. This will be described in detail below.

FIG. 1 shows a state in which a laser beam having two wavelengths λ ₁ and λ ₂ is incident on a film F having a thickness h.
In the figure, λ ₁ and λ ₂ are shown as separate light beams, but this shows the same path as another path for convenience. Focusing on the laser beam L ₀ (λ ₁ ) having the wavelength λ ₁ , this is divided into the component L ₁ (λ ₁ ) that has directly transmitted through the film and the component L ₂ (λ ₁ ) that has been transmitted after being reflected once. Be divided. Furthermore, there is a component that is reflected and then transmitted again, but the influence on the interference fringes is extremely small, and therefore its description is omitted. And L ₁ (λ ₁ )
Interference changes the sum of the light amount of both the between for optical path difference δ which (lambda ₁₎ is changed by the scanning position, L _{1 (λ 1)} and L ₂ (λ ₁₎ and L ₂ (λ ₁₎ and However, interference fringes are generated. Now, exactly the same phenomenon occurs for laser beams L ₂ (λ ₂ ) having different wavelengths λ ₂ . If incident light L ₀ (λ ₁ )
When the optical path difference δ (λ ₁ ) with respect to is an integer multiple of the wavelength λ ₁ ,
Optical path difference δ (λ ₂ ) (= δ for L ₀ (λ ₂ ).
(Λ ₁ )) differs from the integral multiple of the wavelength λ _{2 by} a half wavelength, that is, the phase difference α between the two When π is different from each other, the interference fringes of the two act so as to cancel each other. Therefore, if the above conditions are provided, the interference fringes will be canceled out, and good image reading can be obtained.

Next, the above conditions will be calculated. L ₀ (λ ₁ ) has a thickness h = 17
It is assumed that a film having a thickness of 5 μ and a refractive index n = 1.52 is vertically incident on the film. The optical path difference δ between L ₁ (λ ₁ ) and L ₂ (λ ₁ ) at this time is δ (λ ₁ ) = 2nh · COS θ ′ where θ ′ is the incident angle of the laser beam inside the film.

Becomes Applying a specific numerical value in the above equation, δ (λ ₁ ) =
It becomes 532 μm. L ₀ (λ ₁ ) laser beam wavelength λ ₁
Is 0.8 μm, the phase difference is Becomes That is, the light flux of the light beam and L ₁ (λ ₁₎ of the L ₁ (λ ₁₎ will be is offset by exactly 665 wavelength. When a light flux of wavelength λ ₂ passes through the same optical path difference, L ₁ (λ ₂ ) and L ₂ (λ
If there is a 664.5 wavelength difference with ₂ ), it means a half wavelength difference. Therefore, If so, the interference fringes are offset. This condition is not limited to when the phase differences α (λ ₁ ) and α (λ ₂ ) of _two light beams having different wavelengths are deviated from each other by π, but in reality, | α (λ ₁ ) −α (λ ₂ ) | = Π + 2qπ (q = 0,1,2 ...). Therefore, when λ ₂ is λ ₂ = 0.8 ± (2q + 1) * 0.0006 μm, the interference fringes are canceled.

Similarly, when λ ₂ is calculated for the case of not vertical incidence,
When L ₀ is incident on the film at 24 degrees, δ = 512.6 μm, and eventually λ ₂ = λ ₁ ± (2q + 1) * 0.00062 μm.

FIG. 2 shows the calculation result of the interference fringe reduction effect,
FIG. 7 is a characteristic diagram in which a scanning beam is scanned from 0 to 24 degrees with λ ₂ = 0.80061 μm, taking a distance between the case of vertical incidence.
In the figure, the horizontal axis represents the scanning angle, which represents the angle of incidence on the film surface. That is, the incident angle of 0 degree means a state in which the scanning beam is applied to the central portion of the image. The vertical axis shows the contrast of the interference fringes.
There is no interference fringe reduction effect at the time of, and it completely cancels out at the time of zero. As is clear from the results of FIG. 2, the interference fringes are suppressed to almost zero over the entire scanning width.

As described above, the interference fringes can be almost eliminated by forming the laser beams incident on the film by the lasers having wavelengths λ ₁ and λ ₂ which are slightly different from each other.

In the above example, the effect can be obtained if λ ₂ ≠ λ ₁ ± 0.0012 μm, and more generally, Λ ₁ and λ ₂ satisfying the above are optimal. vice versa, There is no effect at all when λ ₁ and λ ₂ .

In addition, when the incidence is not vertical, each of these conditions Produces the optimum effect with There is no effect when. In consideration of these conditions, λ ₁ and λ ₂ may be set appropriately from the use conditions (film, incident angle, refractive index, etc.). If you cannot select the optimum conditions If so, there is an effect of suppressing interference fringes as compared with the case of performing film reading with a single wavelength.

-Example-Hereinafter, the Example of this invention is described in detail.

FIG. 3 shows the main structure in FIG. 12, in which the semiconductor laser 21 as a light source, the beam shaping lens 22, the polygon 23, and the imaging lens 24 are used to scan the laser beam onto the film 25. 21 is a multi-mode semiconductor laser available as, for example, "LT022, LT023" laser manufactured by Sharp Corporation. Since the semiconductor laser 21 has a beam output having a plurality of wavelengths as shown in the spectrum of FIG. 4, it is possible to obtain an image reading result in which interference fringes are suppressed from the phase difference due to the difference in wavelength. it can. That is, although the wavelength difference Δλ of this multimode semiconductor laser is about 0.3 nm, the interference fringe contrast due to these beam outputs is remarkably reduced, as can be understood from FIG.
Here, the contrast of the interference fringes is not completely 0, but if the interference fringes are of this level, there is no visual problem at all. In addition, in the above-mentioned multimode semiconductor laser, even if operated at an output lower than the rated output, multiple beam outputs with a wavelength difference Δλ of each beam output of about 0.3 nm can be obtained, so similar results Is obtained.

In the above-described embodiments, the semiconductor laser 21 may be configured to obtain a beam including different wavelengths by using a single mode semiconductor laser instead of the multimode laser. Figure 6 shows, for example, "LT02" manufactured by Sharp Corporation.
7) shows the change of each spectrum when the output is changed by the single mode semiconductor laser available as “7”, and the outputs of the single mode semiconductor lasers with the rated output of 3 mW and 5 mW are sequentially reduced to the same figure (a) ~ ( Since the output has a plurality of wavelengths and the wavelength difference Δλ between these beam outputs is about 0.3 nm as shown in d), the interference fringe reduction effect can be obtained as in the above-described embodiment. FIG. 8 shows the contrast characteristic.

Further, there is a single-mode semiconductor laser that oscillates at a plurality of wavelengths when there is return light without lowering its output. The effect can be obtained.

Further, by switching the output of the single mode semiconductor laser at a cycle higher than the scanning pixel frequency, it is possible to read an image with different wavelengths. For example, a semiconductor laser with the characteristics shown in Fig. 7, which is available as "LT026" manufactured by Sharp Corporation, has an output of 5 mW as shown in Figs.
And 3 mW, there is a wavelength difference of about 0.63 nm, so it takes 5 m while the scanning beam reads one pixel.
By switching the power between W and 3 mW, it is possible to obtain the same operation and effect as reading with a laser beam having two wavelengths having the same wavelength difference. FIG. 9 shows a power switching waveform, showing that the switching period of the laser output is changed to a frequency four times higher than the image clock, and this switching is realized by laser output modulation. The contrast characteristic at this time is shown in FIG. In the figure, the interference fringes are reduced to about 0.3, but this is because the powers of two different wavelengths are different, and can be improved by changing the duty ratio when modulating the laser. However, in practice, this degree of improvement is sufficiently effective.

FIG. 11 shows another embodiment. The difference from FIG. 3 is that two semiconductor lasers are used and the respective output wavelengths are made different. The outputs of the semiconductor laser 21 and the beam shaping lens 22 are combined by the half mirror 28 with the outputs of the semiconductor laser 26 and the beam shaping lens 27, and a laser beam having two wavelengths is incident on the polygon 23. In this case as well, it is apparent that the effect of reducing interference fringes can be obtained as in the above-described embodiment.

The lasers 21 and 26 may be different models,
The same model may have different oscillation wavelengths due to manufacturing variations. Further, the synthesis of both beams is not limited to the one using the half mirror, and a configuration in which the semiconductor lasers are arranged in parallel without overlapping two light fluxes into one may be different. Anything can be used as long as the scanning by wavelength can be obtained. Further, it is possible to use a semiconductor laser array to obtain beams having different wavelengths.

-Effects of the Invention-As described above, according to the present invention, since a film image is read by a laser beam having a plurality of wavelengths, a beam directly transmitted through a film image and a beam reflected at an interface on the opposite side are used. There is an effect that the phase difference of 1 can be canceled by the difference in wavelength, and the interference fringes can be effectively reduced.

[Brief description of drawings]

FIG. 1 is a principle explanatory view of the present invention, FIG. 2 is a contrast characteristic diagram of interference fringes according to the present invention, FIG. 3 is a configuration diagram showing an embodiment of the present invention, and FIG. 4 is a laser in FIG. Of FIG. 5, FIG. 5 is a contrast characteristic diagram in FIG. 3, FIGS. 6 and 7 are spectra of a semiconductor laser,
FIG. 8 is a spectrum of an embodiment of the present invention, FIG. 9 is an output modulation waveform diagram of a laser of another embodiment of the present invention, and FIG.
FIG. 11 is a spectrum in FIG. 9, FIG. 11 is a block diagram showing another embodiment of the present invention, FIG. 12 is a block diagram of the image reading device, FIG. 13 is a sectional view of the film, and FIG. And FIG. 15 is a diagram showing the optical path difference of reflected light, and FIG. 15 is a characteristic diagram of the generation order of interference fringes due to the optical path difference. 21,26 …… Semiconductor laser, 22,27 …… Beam shaping lens,
23 …… Polygon, 24 …… Coupling lens, 25 …… Film,
50 ... Ultrasonic light modulator.

Claims (6)

[Claims] 1. An image reading apparatus for scanning a surface of a transparent medium having an image recorded thereon with a laser beam having a plurality of wavelengths, and reading the image from transmitted light refracted at the front and back boundary surfaces of the medium. When the refractive index of the medium is n and the thickness of the medium is h, the wavelength difference between the laser beams having a plurality of wavelengths λ ₁ and λ ₂ is (Where q = 0, 1, 2 ...) The image reading device. 2. The image reading apparatus according to claim 1, wherein the laser beam generating means is a multimode semiconductor laser. 3. The image reading apparatus according to claim 1, wherein the laser beam generating means is a multi-mode semiconductor laser and operates at an output lower than the rated output. 4. The image reading apparatus according to claim 1, wherein the laser beam generating means is a single mode semiconductor laser and is capable of injecting light returned to the semiconductor laser. Reader. 5. The image reading device according to claim 1, wherein the laser beam generating means is a single mode semiconductor laser and changes its output at a frequency higher than the pixel pitch. Image reading device. 6. The image reading apparatus according to claim 1, wherein the laser beam generating means is composed of a plurality of semiconductor lasers having different generation frequencies, and an image is scanned by a combined output of these semiconductor lasers. Characteristic image reading device.