GB2023812A - Photometer - Google Patents

Abstract

A photometer for directionally- independent evaluation of illumination from all directions of incidence from a half space or for evaluation of illumination from light incident at an angle alpha by the function 1/cos ??? up to an upper limit, wherein a light dispersing dome 2 is located above a detector 1, and comprises two superposed body parts 3,4 the upper 3, acting, in use, as a light diffuser without attaining ideal half space geometry and the lower, 4, compensating for deficiencies in the action of 3. Segment 5 is clouded. In the plane of contact 6 an annular diaphragm 10 shades part 4. In an embodiment in which the hemisphere and cylinder have the same diameter the disphragm in plane 6 surrounds them. In another embodiment parts 3 and 4 are thick walled transparent bodies with a thin light scattering coating. <IMAGE>

Description

SPECIFICATION Photometer The present invention relates to a photometer with directionally independent evaluation of illumination from all incidence directions from the half space or with the evaluation of the incident light by the function 1/cos a (a being the angle of incidence) to an upper limit, whereby the light is detected by a detector.

A photometer is known which permits measure mentsto be effected in limonology utilising simple selenium photoelements. The spectral sensitivity of the photoelements is corrected by a filter. However, they exhibit no universal light sensitivity. It has, however, been generally accepted that for limonological purposes, illumination measurement must be effected with a receiver which is uniformly sensitive in all directions.

A further known photometer, which is intended to detect uniformly all radiation in the upper half space, has a simple scattering hemisphere located over the receiver. This arrangement, however, has inherent faults which may give errors in evaluation of up to 100% of the 90" value.

A metering device intended to detect light independently of direction utilising a scattering hemisphere with central screening is also known. This arrangement also gives rise to inaccuracies dependent upon the direction of incidence of the light.

A measuring device for the numerical determination of volume brightness is also known in which the measuring device comprises a solid or hollow sphere which is exposed to the light. The sphere is clouded and serves to collect the light from different directions, so that the integrated brightness of the light fluxes may be measured. This apparatus, although providing adequate directionally independent measurements, is extremely unwieldy, because the sphere must be larger than the light receiver itself. Moreover, such apparatus does not allow the illumination of the upper half space (a solid angle of 2 z) to be detected alone. The separation of the light into upper and lower half-space portions is, however, not strictly essential for the purpose of practical measurement.

In a further known device, a clouded translucent cone with a 340 point angle is mounted on the light receiver. The ground outline and elevational surface area are of the same size in such a cone. Although such a device evaluates light impinging vertically and horizontally thereon equally strongly, it excessively over-evaluates intermediate angles.

A still further known photometer does not directionally independently evaluate the illumination but does provide spectral resolution by the use of a rectilinear sky filter-monochromator.

Most known instruments for directionally independently photometry thus over-evaluate the intermediate angles. In instruments without spectral resolution, complete correction, especially the transition of energy-correct to quantum-correct signals, is made extremely difficult due to the limited choice of suitable filters and receivers. In instruments with monochromators, any desired spectral evaluation is only possible only subsequent to the evaluation of the illumination. Portable measuring instruments which provide spectral resolution and have a watertight housing are inconveniently heavy and unwieldy. Mechanical centre of gravity changes manifest themselves in passing through the monochromator.

In displaceably suspended instrument, such as those used for under-water measurements, this causes a direction change of the measuring head.

The present invention seeks to provide a photometer which makes possible a constant evaluation of the illumination from all directions of incidence of either daylight or of artificial light in the half space.

According to the present invention, there is provided a photometer for directionally-independent evaluation of illumination from all directions of incidence from a half space or for evaluation of illumination from incident light having an angle of incidence a by the function licos a up to an upper limit, whereby the light is detected by a detector, wherein a dome acting as a light dispersing body is located above the detector, the dome comprising two superposed body parts, the upper body part acting, in use, as a light diffuser without attaining ideal half space geometry and the lower body part compensating for deficiencies in the action of the upper body part.

Preferably, colour filters are located between the detector and the dome so as to influence the spectral sensitivity thereof.

Advantageously, the upper body part is hemispherical and has a central, clouded cover portion, the lower body part is a cylinder having a diameter equal to or less than the diameter of the upper body part, such diameters being measured in the plane of contact of the two body parts. Other shapes of the upper body part, such as cones or ellipsoid spheroids, are, however, also possible.

Further desired features of the invention are that screens or diaphragms or opaque surfaces may be arranged in the plane of contact of the two body parts, and an inlet gap in which sky filter monochromator is located may be provided between the detector and dome. In this latter case the monochromator is desirably annular.

A photometer in accordance with the present invention permits illumination and reflection measurements, particularly for ecological, horticultural orwork-physiological purposes, including spectral distribution, to be effected. Thus, over a measurement aperture, a dome of light diffusing material and screens or diaphragms are mounted which have universal sensitivity. The dimensions of the hemisphere or cone and cylinder are selected optimally to achieve a minimum deviation from the directional independency. This also applies to the inverse cosine evaluation, in which the body parts and diaphragms or screens have different dimensions.

More particularly, the diameter of the cylinder may be kept constant for both applications.

The cosine plotting is preferably produced by a flat scatter disc which is known per se and having a defined rim. The spectral resolution is, on the other hand, attained by utilising an annular interference sky filter (sky filter-monochromator) with additional openings for correction filters for integral measurements (white light) or light intensity measurement (lux evaluation). The different spectral evaluation graphs are obtained by suitable wavelengthdependent modification of the electrical output signal of the receiver. Moreover, this makes possible a digitalisation and subsequent conversion by means of a microprocessor accommodated in the apparatus.

The invention will be further described, with reference to certain embodiments thereof, with reference to the accompanying drawings, in which: Figure 1 shows, schematically, a directionally dependent light receiver.

Figure 2 shows, schematically, a directionally independent light receiver.

Figure 3 shows, schematically, how the angle of incidence of light affects the illumination produced.

Figure 4 shows, schematically, the maximum critical angle.

Figures 5, 6 and 7 show, schematically, elevational views of three different embodiments of domes forming part of a photometer in accordance with the present invention.

Figure 8 shows a sectional view through a photometer in accordance with the present invention, and Figures 9 and 10 each show graphs in which the measurement deviation from the ideal is plotted against the angle of incidence.

Conventional illumination measurements require a measuring device in which a light-sensitive element or detector 1 is substantially flat and hence evaluates the illumination incident thereon from different directions in accordance with Lambert's cosine law. This is schematically shown in Figure 1.

The values (M) are plotted for different angles of incidence, M being equal to L cosa where a is the angle of incidence and L is the light intensity. aG is the limiting angle for the light intensity.

Plants are, however, directionally independent light receivers (directional receivers such as detector 1 according to Figure 2). To obtain suitable evaluation of the illumination from different directions for horticultural or ecological purposes, a directionally independent measurement has to be effected, as shown in Figure 2. The detector 1 embraces the entire upper half space (a solid angle of 2is). The measuring value M corresponds to the light intensity L. If measurements in the lower half space are necessary, then either a second measuring device is used or the measurement is repeated by correspondingly rotating the measuring head 1. To ensure shadowless illumination of work places, directionally independent evaluation of the illumination is necessary.If the evaluation is effected utilising the cosine law, the light which illuminates the lateral surfaces of an object remains almost unmeasured. In reflection measurements, on the other hand, light reflected from a surface is detected by a light meter 1 mounted over the surface. For perspective reasons, extreme lateral surface areas AR2 (see in this connection the schematic views for the different directional evaluations of Figure 3 and Figure 4) are evaluated too low by a factor of 1/cos2a compared with the surface area AR1. The receiver 1 then has to compensate for this error by a 1/cosa-characteristic up to a certain critical angle aG.

This shows that light having different incident directions must be evaluated either in accordance with Lambert's cosine law, directionally dependent or inversely to the cosine law (for reflection measurements).

The spectral evaluation is obtained during reflection measurements from the spectral distribution of the incident light and the reflected light according to the equation R(k) = Z()/L(X), wherein L(X) is the spectrum of the light source, R(x) is the reflection spectrum and Z(X) is the spectrum of the reflected light. Frequently, particularly in an open environment L(k) is not known. It therefore must be ascertained shortly before measuring Z(X). This may be carried out with the same instrument. The measuring device has to be suitable for field use and, under certain circumstances, for underwater measurements.

Figures 5 to 7 show three alternative embodiments for making a dome 2, which is suitable for the measurement of illumination of work places with spectral evaluation according to the sensitivity curve for eyes as well as reflection measurements and the measurement of the spectral curve as energy per wavelength interval, as in light energy measurements, and in quanta per wavelength interval, such as, for example, for photochemical purposes in the photosynthesis in plants. The dome 2 shown in embodiment of Figure 5 comprises two body parts 3 and 4, the upper body part 3being a partially covered hemisphere 3 and the lower body part 4 being a cylinder. The diameter of the cylinder 4 is smaller than the diameter of the hemisphere 3 in the plane of contact 6 of these two body parts.

In FigureS, the upper hemispherical body part 3 and the cylinder 4 have the same diameter in the plane of contactS. The upper segment-like part 5 of the part3 is, as in the embodiments shown in Figures 5 and 7 clouded. In the plane of contactS, an annular screen or diaphragm 7 is provided which has larger diameter than that of the two body parts 3 and 4. This screen or diaphragm 7 serves to shade parts of the cylinder 4 from incident light impinging thereon from certain directions. In the embodiments shown in Figures 5 and 7, this shading is taken over by an annular connecting part 10 interconnecting the body parts 3 and 4. The ring 10 is also opaque.

The embodiment shown in Figure 7 is similar to that shown in Figure 5, but the two diffusing body parts 3 and 4 are each formed by providing a thin light-scattering coating on a thick-walled transparent body. The upper body part 3 is a hemisphere and carries the diffusion layer on its external surface,the lower body part 4 is a cylinder and carries the diffusion layer on its internal surface.

Figure 8 shows a photometer, which is also suitable for under-water use. The dome 2 is of the type shown in Figure 7, is watertight and is located on a housing 11. In the diaphragm aperture 12 of the measuring device 11, afilter 13 is located. Below the inlet aperture 12, and within the device 11, a collimating inlet diaphragm 8 is provided. This diaphragm 8 is located over a sky filter monochromator 9, which latter is mounted on a ring 14. The ring 14 is driven by a stepping switch motor 16 through a reduction gear system 15. The detector 1, together with the sky filter monochromator 9 and its ring hoider 14 is mounted in a housing 17. This housing also acts as a holder for the stepping motor 16 and a mounting fora pivotal axis 18 about which the disc 14 rotates.The device is mechanically balanced so that no weight displacement occurs during rotation of the disc 14. The device may therefore be used in displaceable suspension (under water).

Figure 9 shows the deviation A of the actual evaluation from the ideal evaluation of a scatter body 2 in accordance with the embodiment of Figure 5 for 1/cosa-evaluation. The deviation A is shown as function of the angle of incidence a from 0" to 90 .

The ordinate is marked in degrees corresponding to the angle of incidence and a column headed "Delta A" gives a numerical value for the deviation from ideal evaluation curve as a fraction of 1. The dimensions of the dome in accordance with Figure 5 in terms of the coupling radius amount are as follows: Clouded covering above a height of 0.476, the radius of the cylinder 4 is 0.625 and the height of cylinder 4 is 0.532.

Figure 10 shows the calculation of the scatter body 2 with linear evaluation. A shows the deviation of the actual evaluation from the ideal evaluation in fractions of 1 as a function of the angle of incidence. The ordinate is marked in degrees and the column "Delta A" has the same significance as in Figure 9. The dimensions of the dome 2 in terms of the coupling radius are as follows: Clouded covering from a height of 0.698, the radius of the cylinder 4 is 0.625 and the height of the cylinder 4 is 0.202.

Claims (7)

1. A photometer for directionally-independent evaluation of illumination from all directions of incidence from a half space orfor evaluation of illumination from incident light having an angle of incidence a by the function 1/cosa up to an upper limit, whereby the light is detected by a detector, wherein a dome acting as a light dispersing body is located above the detector, the dome comprising two superposed body parts, the upper body part acting, in use, as a light diffuser without attaining ideal half space geometry and the lower body part compensating for deficiencies in the action of the upper body part.

2. A photometer as claimed in claim 1,wherein colour filters are located between the detector and the dome so as to influence the spectral sensitivity thereof.

3. A photometer as claimed in claim 1 or 2, wherein the upper body part is hemispherical and has a central, clouded cover portion, the lower body part is a cylinder having a diameter equal to or less than the diameter of the upper body part, such diameters being measured in the plane of contact of the two body parts.

4. A photometer as claimed in any preceding claim wherein diaphragms or screens or opaque surfaces are arranged in the plane of contact of the two body parts.

5. A photometer as claimed in any preceding claim wherein an inlet gap is defined between the detector and the dome, a sky filter monochromator being located in said gap.

6. A photometer as claimed in claim 5, wherein the sky filter monochromator is annular.

7. A photometer constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.