WO1994000742A1

WO1994000742A1 - Light measurement

Info

Publication number: WO1994000742A1
Application number: PCT/GB1993/001356
Authority: WO
Inventors: Torstein Seim
Original assignee: Nycomed Pharma AS
Current assignee: Takeda AS
Priority date: 1992-06-29
Filing date: 1993-06-29
Publication date: 1994-01-06
Anticipated expiration: 1994-12-29
Also published as: AU4506393A; GB9213737D0

Abstract

There is provided an apparatus for the measurement of reflected light comprising a low intensity light source (2) such as an LED and a light-dependent detection means (3) such as a photodiode, together with capacitative means (4) and means for charging said capacitative means (4) to a predetermined voltage, means for discharging said capacitative means through the light-dependent detector means (3) and means (6) for measuring the rate of discharge of said capacitative means.

Description

Light Measurement
This invention relates to an apparatus and method for carrying out light measurement, and in particular for measuring intensity of reflected light.

In reflectometers and similar apparatus for measuring the intensity of reflected light, an internal light-emitting means is provided in association with light-detection means so that a standard level of illumination is available. In larger reflectometers, relatively powerful light sources may be used, for example lasers, but for some uses, where portability of the device is important, a less powerful light source such as a light emitting diode (LED) is more appropriate.

However, when using LED's and similar low intensity light sources, in relatively simple hand held devices, it is important to use a sensitive means of lightdetection which can readily be miniaturised into a compact form.

Many different types of electrical or electronic components are known, a characteristic of which, e.g.

the resistance, varies in response to the light incident on the component, or a current is generated due to the illumination. As examples one can cite phototransistors, which are mostly used for detecting a light intensity above or below a certain threshold, and photodiodes which are used to determine a light of varying intensity. A number of circuit configurations are known which permit the light-dependent characteristic of such a component, e.g. a photodiode, to be exploited in order to produce a measurement of light intensity.

Fig. 1 illustrates one such known technique in which the photodiode D1 is connected to the inverting input of a conventional operational amplifier. In this way the current generated in the photodiode by light incident thereon is converted to an output voltage VET which may be used as a measurement of the light intensity. This method is satisfactory up to a point, but it suffers from the disadvantage that the current in the photodiode is low and requires high amplification.

The need for high amplification introduces problems such as instability and drift, mainly due to temperaturerelated effects.

In an alternative approach, used for example in video-cameras (Fig. 2), the photodiode D2 is first charged to an initial voltage. Light incident on the photodiode D2 causes a current to flow that discharges the photodiode and reduces the voltage across it. This voltage is then transferred to the output of a Sampleand-Hold circuit (S/H). This constant output voltage may then be converted to a numerical value by an AD converter and processed in any chosen manner. Fig. 2 represents a simplified version, since a video-camera will have many photodiodes. A disadvantage of this method, however, is that the act of sampling may introduce noise into the signal. S/H circuits and AD converters also increase the cost of the equipment.

Thus there remains the need for a simple, effective and compact circuit arrangement for reliably measuring intensity of reflected light to a high degree of accuracy when the target is illuminated by a low intensity light source such as an LED.

Viewed from one broad aspect the present invention provides apparatus for the measurement of reflected light comprising a low intensity light source and lightdependent detection means together with capacitative means, means for charging said capacitative means to a predetermined voltage, means for discharging said capacitative means through the light-dependent detection means, and means for measuring the rate of discharge of said capacitative means.

With this arrangement, the intensity of the incident light is measured by detecting the rate of discharge of the capacitative means, which rate of discharge depends on the current flowing through the light-dependent detection means, which in turn depends on the intensity of the light thereon. This permits a simple, reliable and accurate determination of the light intensity to be made.

The low-intensity light source is preferably a light emitting diode (LED). These have the advantage of small dimensions, particularly suitable for hand-held reflectometers.

The light emission of LEDs and, indeed, some other light sources is linearly but inversely dependent on temperature over the range 0 to 100 C. While variations in light emission can in part be compensated by a control reading of a surface of standard reflectance, it is advantageous to provide a light source having reduced variability with temperature. It is thus advantageous to connect the LED to-voltage correction circuit adapted to compensate for temperature variations. This can be achieved using a voltage regulator to supply a constant voltage to the LED via a resistor. We have found that an LED can be regarded as equivalent to a Zener diode in series with a resistor.

The voltage across this Zener diode is constant for constant temperature but decreases with increasing temperature and the light output of the LED will also be decreased. This light output decrease will be partially corrected by keeping the current through the LED constant. However by appropriate setting of the voltage regulator connected to the LED via an appropriate resistor, the light output of the LED can be stabilised.

The discharge rate of the capacitative means may be determined in a number of ways, but it is preferred to provide comparing means whereby the time may be measured until the voltage across the capacitative means has fallen to a predetermined level. A differential amplifier may serve as such a comparator with the voltage across the capacitative means being one input and a reference voltage being the other input.

The light-dependent element is preferably a photodiode whose conductance varies in response to an incident light, but any component may be employed that has an electrical or electronic characteristic that varies in response to incident light, e.g. a phototransistor, a photovoltaic element, a photoresistor or an equivalent.

Viewed from another broad aspect the present invention provides a method for measuring reflected light intensity comprising illuminating a target surface with light from a low intensity light source, detecting light reflected from said surface with light-dependent detection means by charging capacitative means to a predetermined voltage, discharging said capacitative means through said light-dependent detection means, and measuring the rate of discharge of said capacitative means.

Preferably, the rate of discharge is determined by measuring the time taken for the voltage across the capacitative means to fall to a second predetermined value.

The present invention has a wide range of possible applications. A particularly preferred application, however, is as a reflectance meter for determining the colour and/or intensity of biomedical test cards which are subject to a colour change on application of a test sample depending on the contents of the test sample.

It should also be understood that although reference is made in this specification to the term "light", it is not intended that the invention be limited to visible light, but rather the invention may also extend to the non-visible parts of the electromagnetic spectrum.

An embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which:
Figs. 1 and 2 are circuit diagrams illustrating prior art arrangements as discussed hereinabove;
Fig. 3 is a circuit diagram showing an embodiment of the light detection system of the invention;
Fig. 4 is a graph showing measurement results of voltage drops across the capacitative means for different light intensities; and
Fig. 5 is a circuit providing stabilised light output from an LED.

Referring to Fig. 3, a target surface 1 is illuminated by an LED 2 and reflected light from the surface 1 falls on a photodiode 3. One side of capacitor 4 is earthed, the other is connectable via switch 5 to a source of a first predetermined voltage
Vo. Closing switch 5 causes capacitor 4 to be charged to voltage -Vo. Connected in parallel with capacitor 4 is the photodiode 3, the photodiode 3 being connected across the capacitor in the non-conducting direction relative to Vo polarity. The capacitor voltage is provided as the non-inverting input to a differential amplifier 6 acting as a comparator. A second predetermined reference voltage Vt is provided to the other, inverting, input of the comparator.

In operation, switch 5 is firstly closed to allow the capacitor 4 to charge to Vo. When constant reflected light is incident on the photodiode 3 to provide a constant value for the conductance and the switch 5 is opened, the capacitor discharges through the photodiode linearly with respect to time, the rate being proportional to the conductance and thus to the incident light intensity, as demonstrated in Fig. 4. The rate of discharge is detected by measuring the time taken for the capacitor voltage to fall to voltage Vt supplied to the other input to the comparator 6, such measurement being made by any suitable conventional timing technique.

One possible application for the measurement circuit described above is for use in conjunction with a biomedical solid phase immunoassay test card, for example when using light measurement apparatus of the type disclosed in our co-pending application of even date entitled "Light Measurement Apparatus" the contents of which are incorporated herein by reference. Such test cards are designed to indicate the presence of a compound, e.g. a protein, in a liquid, e.g. blood or serum. A card is prepared with one or more test sites which change colour to a varying extent depending on the presence and amount of the particular compound concerned. Measurements of actual protein concentration can be made to a certain extent simply by eye by comparing the test site with a reference colour chart.

This is, however, clearly unsatisfactory for accurate readings, and the present invention may be applied to an automated system in which the colour change is detected by measuring the reflectance of the test site.

By measuring the time T for the voltage to be reduced to Vt, the light intensity L is given by
L = - C being a constant.

CT
When measuring reflected light from a sample, the reflectance r may easily be calculated by comparing the reflected light from the sample Ls with reflected light from a reference Lr;
r = Ls/Lr = Tr/Ts where Tr and Ts are measured time for Lr and Ls respectively.

Possible sources of error in this technique are dark current Ld in the photodiode, and scattered and/or reflected light Lc from other components, but these possible errors can be taken into account. The measured light Lm will consist of
Lm = Ls + Ld + Lc, where Ld can be measured with the light source switched off, and Lc can be measured with the light on but the sample removed.

One of the advantages of the present invention over the prior art is that no additional equipment is required to convert the light intensity to a numerical value. Converting from time T to a number is done by the micro-computer in the apparatus simply by counting clock pulses generated by the processor's crystal oscillator. The high impedance of the capacitor/diode circuit, combined with a high input-resistance amplifier, will reduce temperature drift due to thermic resistance changes in the equivalent-resistance of the diode. However, the diode should have low dark current, compared to the light current used, since the dark current will vary with temperature. If photodiodes with 1d of about 20*10-12 A, compared to Ir of about 10-6 A, the dark current will lead to insignificant errors.

Due to the small dimensions of modern LEDs and photodiodes, these can be placed close to the sample as done in the light measurement apparatus described in our co-pending application of even date, and thereby minimising the error due to 1d since 1r will be sufficiently large even when LEDs are used as light sources.

Fig. 5 shows a circuit in which an adjustable voltage regulator U1 is supplied with an input voltage
V-IN and capacitors C1 and C2 and resistors R1 and R2 to provide a stabilised output voltage which is passed through a resistor R3 to the LED. As the voltage across the LED decreases with increased temperature, the voltage drop across the resistor R3 changes to produce an increased current across the LED, thereby countering any decrease in light output due to temperature.

The invention provides advantages of simplicity in contrast to the prior art, enabling accurate measurements with reduced errors from, e.g. temperature changes. These advantages may be obtained without requiring high-intensity light sources.

Claims

1. Apparatus for the measurement of reflected light comprising a low intensity light source (2) and lightdependent detection means (3), together with capacitative means (4) and means for charging said capacitative means (4) to a predetermined voltage, means for discharging said capacitative means through the light-dependent detector means (3) and means (6) for measuring the rate of discharge of said capacitative means.

2. Apparatus as claimed in claim 1 in which the low intensity light source is stabilised.

3. Apparatus as claimed in claim 1 or claim 2 in which the low intensity light source is a light-emitting diode.

4. Apparatus as claimed in claim 3 in which the lightemitting diode is connected to a voltage correction circuit adapted to compensate for temperature dependent light output variations.

5. Apparatus as claimed in any of the preceding claims in which the rate of discharge of the capacitative means is determined by measuring the time for the voltage across the capacitative means to fall to a predetermined voltage.

6. Apparatus as claimed in claim 5 in which a differential amplifier is used to compare the voltage across the capacitative means with a reference voltage.

7. Apparatus as claimed in any of the preceding claims in which the light-dependent detection means is a photodiode.

8. A method for measurement of reflected light intensity comprising illuminating a target surface with light from a low intensity light source and detecting light reflected from said surface with light-dependent detection means by charging capacitative means to a predetermined voltage, discharging said capacitative means through said light-dependent detection means and measuring the rate of discharge of said capacitative means.

9. A method as claimed in claim 8 in which the light source is stabilised to produce light of constant intensity.

10. A method as claimed in claim 8 or claim 9 in which the target surface is a coloured surface on a biomedical test card.