Note: Descriptions are shown in the official language in which they were submitted.
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~ WO91~195~ PCT/GB91/~9
2084789
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LIGHT ~E~CTOR .
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,", 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 æensors capable of s?nsing
energy in the above regions Or the electroma.Jnetic
spectrum.
Light detectors specifically adapted to detect
and measure W light are known, see for example US
20 Patent 4,372,680; GB Specification 2,181,833A, US
Patent 4,010,372, US Patent 4,065,672, German O~S
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
: 25 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,
~ WO 91/19538 PCI'/GB91/00879
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~ the arrangement being such that one sensor receives
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~ , light of a range of frequencies including a specific
e ~ range of frequencies to be detected and the other of
which receives light of the same range of frequencies as
S 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.
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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
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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.
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According to one aspect of the present
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/091/1ff ~ PCTrGB91/~U~
invention there is provided a detector for measuring the
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intensity of light, as hereinbefore defined, incident
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. upon a surface, comprising a plurality of sensors
adapted to produce a signal related to the intensity of
S 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 relaSion to the
light incident upon them.
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For example, two or more sensors can be
arranged concentrically or as separate entities within
the envelope of a single large sensor.
.
I~ 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
select-d spectral band
The light sensors may be of any appropriate
type and may, for example, be photovoltaic sensors such
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as photodiodes or phototransistors, solar cells,
photoresistors, or photoemissive sensors, such as
photomultipliers.
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5The 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.
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Light sensors are often more sensitive to
Light in some spectral bands than others, and it can
therefore be advantageoùs to provlde further optical
ilters 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.
20Preferably the invention provides a detector
for W -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 W -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
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W091/19538 W GB91/ ~ 79
-~.for combining the outputs from the sensors is arranged :.to subtract the output from the sensors thereby to give
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., .s.. ~ .v~w~J a difference signal which is representative of the
intensity of the W -B light which is incident upon the
. 5 detector.
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According to a further aspect of the present
invention there is provided a detector for measuring the
intensity of light, as hereinbefore defined, comprising
10 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 ~or comblning 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 descrlbed, by way of
example, with reference to the accompanying drawings, in
which:-
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Figure 1 shows diagrammatically an embodiment
of the invention,
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;: Figure 2 is a plan view of the arrangement of
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W091/l9~38 PCT/GB91/00879
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two sensors incorporated in the embodiment of the
,'l; ; ~ ~?invention shown in Figure 1, and
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;Figure 3a to _ show diagra~matically the solar
spectrum, the spectral characteristics of filters used
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;in the embodiment of Figure 1 and how they conbine to
achieve the desired result of rendering the embodiment
of Figure 1 sensitive to W -B light only.
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- - 10Referring to Figure 1, a detector for W -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 sen60rs 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 xnown
- in the semi-conductor device art and which will not be
described further.
25_
Ideally, the sensors 11 and 12 are a matched
pair, so as to have an identical or substantially
; identical response. ?~owever, if they are not a matched
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~j WO 91/19538 PCI`/GB91/00879
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2084789 - :.-;
~ ~ ~ pair, they could be electronically balanced.
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; The filter 13 forms a window in the housing
10, and this filter is of nominal W -transmitting glass
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such as Hoya U-340. This gIass transmits light energy
in both the W -A and W -8 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.
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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 "tran6parent" glass or plastics film,
such as Melinex polyester fllm, having a uniformly high
transmission characteristic for both the W -A and IR
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components of the light falling on it from the filter
13, but a very poor transmission characteristic for the
W -B component of the light.
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lt 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
Cilter 14. This can be achieved by prcviding 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.
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~ WO 9V19S38 PCI'/GB91/0087g
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The output signals of the two sensors 11 and
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, ,,~ ;, 12 are subtracted by applying these output signals in
'.t'~ ySI~ opposition to one another across opposite ends of the
centre-tapped resistor 15, or other electronic
subtraction device.
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The operation of the detector of Figure 1 will
now be explained with references to-Figure 2a to 2e.
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A typical spectrum for light incident on the
filter 13 is shown in Figure 2a. This includes light
energy within the W , 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 2k. This consists
o~ light energy in both the W -A and W -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
e~ergy falls directly on the sensor 11. The photo-
sensitivity of silicon is signlficantly greater in redand near-IR than at the W 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 W -A component of
the light transmitted by the filter 13 and also the
small amount of IR radiation leaking through filter 13,
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~ ~ ~ ~j WO 91t19538 PCI~/GB91/00879
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as shown in Figure 2d. It will not, however, transmit
;. i . the W -8 component (Yigure 2e) of the light transmitted
.... ,'V by the filter 13. Thus, subtraction of the output
signals of the two sensors 11 and 12 will result in the
S cancellation of the signal components arising from the
, ' ~ ' W -A and IR parts of the incident spectrum so that the
resultant differential signal will be representative of
the W -B part of the spectrum alone.
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, ; ; . . j, 10The filter 13 could be omitted, but it has the
; ? advantage that it removes a significant part of the non-
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W -8 light from the incident spectrum, resulting in more
accurate measurement of the W -B intensity.
15Also, the filter 1~ could be placed in front
; o~ the ~ilter 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 ~orm of silicon photodiodes formed
on a singlo 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) i5 provided on the active area
o~ 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
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~ ~ ~ ~ W091/19538 PCT/GB9t~00879 ~ ~
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~ '' to the direction of the light which is incident upon it.
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i^-- The detector described above is designed to
respond to a single component of the light spectrum.
~owever, 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 W -A, W -B, W -C and
visible bands within the solar spectrum.
In a further embodiment, the detector shown in
Figure l could be modi~ied by providing a third ~ilter
(not shown) between the filter 13 and the sensor ll.
The selection of the transmission spectra of all three
~ilters will give rise to a greater range of
possibilities in the design of the detector, As before,
the ~ilter 13 could be omitted.
The subtraction device lS 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 o~ a person to the risk Or over-exposure to
solar or artificial ultra-violet radiation which could
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~ WO91/19Sa~ -11- PCT/C~91/00~79 ~ ~
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, ;- - cause erythema or even a risk of melanoma.
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~,,"~,* ~ 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
Sorm 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.
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