WO1991019538A1

WO1991019538A1 - Light detector

Info

Publication number: WO1991019538A1
Application number: PCT/GB1991/000879
Authority: WO
Inventors: Bernadette Leanne Armitage Ward; William John Bailey
Original assignee: Uvisol Limited
Priority date: 1990-06-09
Filing date: 1991-06-03
Publication date: 1991-12-26
Also published as: GB9111866D0; GB2245705A; EP0566569A1; CN2098676U; AU658568B2; GB2245705B; AU7903591A; JPH05508911A; CA2084789A1

Abstract

A detector for measuring the intensity of light comprises two sensors (11, 12) adapted to produce output signals related to the intensity of light incident upon them, a filter (14) for attenuating light received by one of the sensors in a selected spectral band, and means (15) for combining the output signals of the two sensors to provide a detector output signal representative of the intensity of the light in the said spectral band incident upon the detector. The sensors (11, 12) are so disposed in relation to one another, for example they may be arranged concentrically, as to render them insensitive to their orientation in relation to light incident upon them. Also, there may be a further filter (13) for attenuating light received by both sensors in a further selected spectral band.

Description

IGHT DETECTOR

The invention relates to a light detector and more particularly but not exclusively to a detector which can detect electromagnetic radiation over one or more selected spectral bands within the ultra-violet (UV) , visible and infra-red (IR) regions of the electromagnetic spectrum.

For the purposes of this specification electromagnetic radiation within these regions of the spectrum shall be referred to as light and references to light should be construed accordingly. Thus the term "light sensor" covers all sensors capable of sensing energy in the above regions of the electromagnetic spectrum.

Light detectors specifically adapted to detect and measure UV light are known, see for example US Patent 4,372,680; GB Specification 2,181,833A, US Patent 4,010,372, US Patent 4,065,672, German OLS 3,042,084, French Patent 2,545,932; US Patent 4,428,050, US Patent 4,485,306, US Patent 4,535,244, US Patent 4,704,535, US Patent 4,851,686, PCT Application WO 90/10201. It is also known from the above documents that a suitable configuration for such detectors is to employ two sensors with matched spectral responses one or both of which has an optical filter in front of it, the arrangement being such that one sensor receives light of a range of frequencies including a specific range of frequencies to be detected and the other of which receives light of the same range of frequencies as the first sensor except that the range of frequencies to be detected is prevented from reaching the second detector. The intensity and/or accumulated dose of the range of light frequencies is obtained by subtracting the output from one sensor from that of the other.

However, in the prior art device which uses more than one optical sensor, these sensors are placed separately, usually side-by-side. As a result, such a multi-sensor device has a response which is sensitive ,to its orientation with respect to the light incident upon it. This can be a disadvantage, particularly in the case of a device which is intended to measure the intensity and/or accumulated dose of ultra-violet light of the type which initially causes tanning but in excess can lead to harmful effects such as erythema or in extreme cases, melanoma. This disadvantage would be particularly apparent if the device were to be in a form such that it could be worn by a user as such a person would be unlikely to remain in one position so that the orientation of the device with respect to the sun would be changing repeatedly.

According to one aspect of the present invention there is provided a detector for measuring the intensity of light, as hereinbefore defined, incident upon a surface, comprising a plurality of sensors adapted to produce a signal related to the intensity of light incident upon them, an optical filter associated with at least one of the sensors and adapted to attenuate light in a selected spectral band incident upon that sensor and means for combining signals from the sensors to provide an output signal representative of the intensity of the light in the said spectral band incident upon the detector wherein the sensors are so disposed in relation to one another as to render them insensitive to their orientation in relation to the light incident upon them.

For example, two or more sensors can be arranged concentrically or as separate entities within the envelope of a single large sensor.

If the detector has two sensors, the detector could have a first optical filter arranged to attenuate light received by one sensor in a first selected spectral band and a second optical filter arranged to attenuate light received by the other sensor in a second selected spectral banc-L

The light sensors may be of any appropriate type and may, for example, be photovoltaic sensors such as photodiodes or phototransistors, solar cells, photoresistors, or photoemissive sensors, such as photomultipliers.

The detector could have more than two light sensors and two or more optical filters for attenuating light in different spectral bands. Such a detector could be arranged to give multiple signals, which could be processed to indicate the light intensity within a number of spectral bands.

Light sensors are often more sensitive to light in some spectral bands than others, and it can therefore be advantageous to provide further optical filters to attenuate the light incident on the detector in spectral bands other than the aforesaid selected band or bands to facilitate accurate detection of light in the selected spectral band or bands.

Preferably the invention provides a detector for UV-B radiation, that is to say, light having wavelengths in the range between 280-315 nm comprising a first filter adapted to attenuate visible and infra-red light but to transmit ultra-violet light, a second filter adapted to attenuate UV-B light only wherein one of the sensors is arranged to receive light transmitted by the first filter and the second sensor is arranged to receive light transmitted by both filters and the means for combining the outputs from the sensors is arranged to subtract the output from the sensors thereby to give a difference signal which is representative of the intensity of the UV-B light which is incident upon the detector.

According to a further aspect of the present invention there is provided a detector for measuring the intensity of light, as hereinbefore defined, comprising two light sensors adapted to produce output signals related to the intensity of light incident upon them, first optical filter means for attenuating light received by one of the sensors in a first selected spectral band, second optical filter means for attenuating light received by both sensors in a second selected spectral band, and means for combining the output signals of the two sensors to provide a detector output signal which varies as the light incident on the detector in the first spectral band varies.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:-

Figure 1 shows diagrammatically an embodiment of the invention.

Figure 2 is a plan view of the arrangement of two sensors incorporated in the embodiment of the invention shown in Figure 1, and

Figure 3a to e show diagrammatically the solar spectrum, the spectral characteristics of filters used in the embodiment of Figure 1 and how they combine to achieve the desired result of rendering the embodiment of Figure 1 sensitive to UV-B light only.

Referring to Figure 1, a detector for UV-B radiation comprises a light proof casing 10, two light sensors 11 and 12, two optical filters 13 and 14 and a subtracting circuit illustrated by a centre-tapped resistor 15 to which the outputs from the sensors 13 and 14 are applied in opposition.

The sensors 11 and 12 are silicon photodiodes, -but any other convenient form of optical sensor could be used. The sensor 11 is in the form of an annulus which surrounds the sensor 12, which is circular. They are mounted on a common insulating substrate 16 and can, if desired, be produced by techniques which are well known in the semi-conductor device art and which will not be described further.

Ideally, the sensors 11 and 12 are a matched pair, so as to have an identical or substantially identical response. However, if they are not a matched pair, they could be electronically balanced.

The filter 13 forms a window in the housing 10, and this filter is of nominal UV-transmitting glass such as Hoya U-340. This glass transmits light energy in both the UV-A and UV-B parts of the spectrum and also a small fraction of the near-IR energy at a wavelength in the region of 700 nm. This energy (for example from the sun) is allowed to fall unimpeded on the sensor 11.

The filter 14 is interposed between the filter 13 and the sensor 12. The filter 14 is in the form of a piece of suitable "transparent" glass or plastics film, such as elinex polyester film, having a uniformly high transmission characteristic for both the UV-A and IR components of the light falling on it from the filter 13, but a very poor transmission characteristic for the UV-B component of the light.

It is important that the sensor 11 should receive light directly from the filter 13 and that the sensor 12 should only receive light transmitted by the filter 14. This can be achieved by providing a non- light transmitting screen 17 between the two sensors, or by providing the filter 14 on the sensor 12 and making the sides of the filter 14 and sensor 12 opaque so as to provide the screen 17 as shown in Figure 1. The output signals of the two sensors 11 and

12 are subtracted by applying these output signals in opposition to one another across opposite ends of the centre-tapped resistor 15, or other electronic subtraction device.

The operation of the detector of Figure 1 will now be explained with references to Figure 2a to 2e.

A typical spectrum for light incident on the filter 13 is shown in Figure 2a. This includes light energy within the UV, Visible and IR ranges of the electromagnetic spectrum. This light is filtered by filter 13 and a typical spectrum for light transmitted by the filter 13 is shown in Figure 2b. This consists of light energy in both the UV-A and UV-B parts of the spectrum and also a small fraction of the near-IR energy at a wavelength in the region of 700 nm. This light energy falls directly on the sensor 11. The photo- sensitivity of silicon is significantly greater in red and near-IR than at the UV end of the spectrum. Consequently, a significant part of the output signal of the sensor 11 will be due to the small amount of IR radiation leaking through the filter 13. The filter 14 .has a transmission characteristic as shown in Figure 2c. The filter 14 will thus transmit the UV-A component of the light transmitted by the filter 13 and also the small amount of IR radiation leaking through filter 13, as shown in Figure 2d. It will not, however, transmit the UV-B component (Figure 2e) of the light transmitted by the filter 13. Thus, subtraction of the output signals of the two sensors 11 and 12 will result in the cancellation of the signal components arising from the UV-A and IR parts of the incident spectrum so that the resultant differential signal will be representative of the UV-B part of the spectrum alone.

The filter 13 could be omitted, but it has the advantage that it removes a significant part of the non- UV-B light from the incident spectrum, resulting in more accurate measurement of the UV-B intensity.

Also, the filter 14 could be placed in front of the filter 13 instead of behind the filter 13.

Figure 3 shows the arrangement of the two sensors. The sensors 11 and 12 shown in Figure 3 are, in practice, in the form of silicon photodiodes formed on a single silicon chip 16. The two photodiodes are provided one within the other and are of similar, preferably equal, surface areas. The optical filter 14 (not shown in Figure 3) is provided on the active area of the inner photodiode 12 as a coating. If desired it can be provided on the active area of the outer photodiode 11. This arrangement of sensors has the advantageous property that the detector is insensitive to the direction of the light which is incident upon it.

The detector described above is designed to respond to a single component of the light spectrum. However, the detector could have more than two light sensors and appropriate filters and such a detector could be arranged to give multiple signals, which could be processed to indicate the light intensity within a number of separate spectral bands. For example, a four- sensor detector could be constructed to give signals which will be proportional to UV-A, UV-B, UV-C and visible bands within the solar spectrum.

In a further embodiment, the detector shown in Figure 1 could be modified by providing a third filter (not shown) between the filter 13 and the sensor 11. The selection of the transmission spectra of all three filters will give rise to a greater range of possibilities in the design of the detector. As before, the filter 13 could be omitted.

The subtraction device 15 may also include an integrator and read-out so that the total energy received in the selected spectral range can be determined and indicated. This is of particular importance if the detector is to be used to monitor the exposure of a person to the risk of over-exposure to solar or artificial ultra-violet radiation which could cause erythema or even a risk of melanoma.

Also, the use of pyro-electric sensors with appropriate filters could result in a detector that will respond to two separate but closely spaced bands in the Infra-red part of the spectrum. The detector would then form the basis of remote surface temperature sensor that would be relatively insensitive to variations in the emissivity of the target surface and thereby give a more accurate estimate of surface temperature than is given by a single wavelength pyrometer.

Claims

CIAIMS

1. A detector for measuring the intensity of light, as hereinbefore defined, incident upon a surface, comprising a plurality of sensors adapted to produce a signal related to the intensity of light incident upon them, an optical filter associated with at least one of the sensors and adapted to attenuate light in a selected spectral band incident upon that sensor and means for combining signals from the sensors to provide an output signal representative of the intensity of the light in the said spectral band incident upon the detector wherein the sensors are so disposed in relation to one another as to render them insensitive to their orientation in relation to the light incident upon them.

'2. A detector according to claim 1, wherein the sensors are optically isolated from one another.

3. A detector according to claim 2, wherein the sensors have equal surface areas.

4. A detector according to any of claims 1 to 3, wherein the sensors are arranged concentrically.

5. A detector according to any proceeding claim, wherein there is provided two sensors and a first -optical filter associated with both sensors and arranged to attenuate equally light in a selected spectral band incident upon both sensors and at least one second optical filter associated with one of the sensors and adapted to attenuate light in one or more selected bands incident upon that sensor.

6. A detector according to any of claims 1 to 5, wherein the optical filters are such that the output signal from the detector is representative of the intensity of UV-B light incident upon the said surface.

7. A detector according to any of claims 1 to 5, wherein there is provided at least three optical sensors and the optical filters are such that output signals representative of the intensities of light in at least two spectral bands are provided by the light detector.

8. A detector according to claim 7, wherein there is provided four optical sensors and the optical filters are such that output signals representative of the intensities of light in the UV-A, UV-B, UV-C and visible bands of the solar spectrum are produced by the light detector.

9. A detector according to any preceding claim, wherein there is included means for integrating the signal representative of the intensity of light in a selected spectral band incident upon the detector and •providing an indication of the total amount of optical energy in that spectral band incident upon the surface.

10. A detector according to any of claims 1 to 5 or claim 7, wherein the sensors are adapted to be sensitive to the infra-red region of the electromagnetic spectrum, and wherein there is provided at least one optical filter such that the optical sensors produce signals representative of the intensity of infra-red light in two different regions of the infra-red band of the electromagnetic spectrum incident upon the detector .and means for combining the signals from the sensors to provide an indication of the surface temperature of a body.

11. A detector for measuring the intensity of light, as hereinbefore defined, comprising two light sensors adapted to produce output signals related to the intensity of light incident upon them, first optical filter means for attenuating light received by one of the sensors in a first selected spectral band, second optical filter means for attenuating light received by both sensors in a second selected spectral band, and means for combining the output signals of the two sensors to provide a detector output signal which varies as the light incident on the detector in the first spectral band varies.