Light detection and control system

An optical detection and control system is described for controlling the brightness of a lamp, or compensating for variations in the brightness of a lamp, such as is used, for example, in a glossmeter, hazemeter, reflectometer, colourimeter or opacity meter. In one system light is transmitted along a path from the light source (1) to a region of interest (8), means (10) are arranged to detect light scattered from the said path, and a power control (12) is coupled to the detecting means (10) to control the brightness of the light source. In another system light is transmitted along a path from the light source (1) to a region of interest (8), a first detector (10) detects light scattered from the said path, a second detector (9) detects light which has reached the region of interest (8), and a signal is produced which is proportional to the ratio of the outputs of the two detectors (8, 9).

FIELD OF THE INVENTION 
This application relates to an optical detection and control system. It is 
particularly concerned with controlling the brightness of a lamp, or 
compensating for variations in the brightness of a lamp, such as is used, 
for example, in a glossmeter, a hazemeter, a reflectometer, a colourimeter 
or an opacity meter. 
SUMMARY OF THE INVENTION 
According to the present invention there is provided a system for 
controlling the brightness of a light source, which comprises an optical 
system for transmitting light along a path from the light source to a 
region of interest, means for detecting light deflected from the said 
path, and control means coupled to the detecting means for controlling the 
brightness of the light source. 
The invention further provides a system for compensating for variations in 
a light source, which comprises an optical system for transmitting light 
along a path from the light source to a region of interest, first 
detecting means for detecting light deflected from the said path and for 
producing a first output in response thereto, second detecting means for 
detecting light which has reached the region of interest and for producing 
a second output in response thereto, and means connected to receive the 
first and second outputs and for producing in response thereto a signal 
which is substantially independent of variations in the light source. 
In preferred embodiments of the invention the above mentioned deflection of 
light is produced by scattering.

THE DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the embodiment of FIG. 1, light from a lamp 1 is incident on a diffuser 
2. The lamp 1 may, for example, be a tungsten filament lamp or a quartz 
halogen lamp. Light passing through the diffuser strikes an image disc 3 
of opaque material, in which there is formed a small slit 4. In an actual 
example of the image disc the slit was 1.152 mm by 0.288 mm, though it 
will of course be understood that other slit sizes could be used. Light is 
scattered from the slit 4 and enters the interior of a cylindrical tube 5 
which may, for example, be of a material such as a black polyacetal resin 
sold under the Trade Mark DELRIN. A substantial portion of the light 
travels along the tube 5 to a lens 6 which forms a parallel beam of light 
7. The beam 7 strikes a surface 8 whose reflective properties it is 
desired to investigate. Light reflected from the surface 8 is detected by 
a suitable detector 9, for example, a photodiode. The output of the 
photodiode can then be analyzed in ways which will be well known to those 
skilled in the art so as to provide information concerning the reflective 
properties of the surface 8. 
The system thus far described is conventional and is the basis of known 
glossmeters. It has been found, however, that a problem arises in the 
operation of such glossmeters in that the amount of light striking the 
slit 4 does not remain constant, and thus spurious variations occur in the 
amount of light detected by the photodiode 9. Variations in the amount of 
light striking the slit 4 arise basically from two causes. Firstly, there 
are long term variations caused by the gradual change in the output of the 
lamp over its life. Secondly, there are changes which can be much more 
rapid and which are due to movement of the filament within the lamp. 
In order to deal with this problem the embodiment of FIG. 1 incorporates a 
further photodiode 10 which is positioned in the sidewall of the tube 5. 
Some of the light scattered from the slit 4 falls on the photodiode 10 and 
the electrical output signal produced thereby is amplified by an amplifier 
11 which is connected to a power control circuit 12 to which the lamp 1 
and its power supply 13 are also connected. The photodiode 10 thus detects 
any reduction or increase in the amount of light reaching the slit 4 and 
the signal from the amplifier 11 to the power control circuit 12 produces 
a corresponding increase or decrease in the amount of power fed to the 
lamp 1 so as to restore the amount of light detected by the photodiode 10 
to its original value. This correction can take place so rapidly as to be 
virtually instantaneous, and thus for practical purposes the amount of 
light reaching the slit can be regarded as being substantially constant 
with time. 
In the embodiment of FIG. 2 a somewhat different approach is adopted. Here 
the light output of the lamp 1 is not corrected, and the output signal 
from the amplifier 11 is fed to a signal processor 14, and the output 
signal from the photodiode 9 is also fed, via an amplifier 16, to the 
signal processor 14. This procedure at its output 15 a signal which is the 
ratio of the output signal of the photodiode 9 and the output signal of 
the photodiode 10, or is proportional thereto. Since both of these outputs 
are equally affected by variation in the lamp 1, their ratio is 
independent of variations in the lamp 1, and thus the circuit of FIG. 2 
provides compensation for any variations in the lamp 1. 
FIG. 3 shows the circuit used in the embodiment of FIG. 2, in slightly more 
detail. It can be seen from this that the power source used is a battery, 
which is connected to a voltage regulator. The output of the regulator is 
a stabilised voltage which is indicated as being, by way of example, five 
volts d.c. 
FIG. 4 shows in considerably more detail the circuitry of FIG. 3, but 
omitting the battery and voltage regulator, which may be of entirely 
standard construction, and also omitting one or two incidental features 
which are mentioned in the description below. 
The voltage produced by the photodiode 9 is supplied to the input of an 
amplifier IC4 via an array of four electronic switches denoted as IC3. The 
purpose of these switches is explained below. The amplified voltage 
produced at the output of amplifier IC4 is supplied by input terminals of 
an integrated circuit IC1 which, as explained below compares this voltage 
with a reference voltage. The circuit IC1 is indicated, by way of example, 
as being that available from Ferranti Electronics Ltd. of Oldham, Lancs, 
England as ZN451E. The circuit IC1 has terminals .phi..sub.1 and 
.phi..sub.2 at each of which appears a square wave signal, the signals 
being 180 out of phase with one another. Thus, considered together, the 
output of the terminals .phi..sub.1 and .phi..sub.2 is a square wave. This 
square wave is applied to the array of electronic switches IC3, so that 
they are alternately switched on in pairs. Thus, at any given instant 
either the inner two switches are conductive and the top and bottom switch 
are nonconductive, or vice versa. The effect of this is that the voltage 
from the photodiode 9 is applied alternately in opposite directions to the 
input terminals of the amplifier IC4. Thus, the voltage supplied by the 
amplifier IC4 to the circuit IC1 is alternately switched, and the circuit 
uses this in order to compensate for any error in the zero position. 
The voltage from the photodiode 10 is amplified by the amplifier IC5, which 
is in two stages. The voltage appearing at the second stage of the 
amplifier IC5 is applied to the input Vr1 of the circuit IC1. This is the 
reference voltage. Logic in the circuit IC1 divides the amplitude of the 
voltage supplied by the amplifier IC4 by the voltage appearing at the 
input Vr1 to give the ratio thereof. An A/D converter within the circuit 
IC1 drives a display which, in this example, is a 31/2 digit display, with 
each digit being represented in conventional 7-bar form. The most 
significant digit can either be 1 or is absent, and the remaining three 
digits can be anywhere from 0 to 9. A signal constantly applied to the 
input DP3 ensures that a decimal point is permanently inserted before the 
least significant digit. A signal from the power supply to the terminal 
labelled LO provides a signal if and when the voltage of the battery which 
provides the power supply falls below an acceptable level, and when this 
signal is provided the display indicates the fact. The back plane signal 
conventionally required for such a display is provided at the inputs BP. 
The remaining terminals of the display are not used. 
The circuit is s arranged that when the lamp 1 is not operating the 
transistor TR3 is turned on via the exclusive-OR gates IC2 so that the 
voltage applied to the reference terminal Vr1 is then zero. For so long as 
the voltage at Vr1 remains zero the circuit IC1 continues to supply to the 
display the last value which was obtained, so that this value continues to 
be displayed. However, as soon as the lamp 1 is turned on, and this is 
done by depressing the switch SW in the direction indicated by the arrow, 
the gates IC2 turn the transistor TR3 off and the voltage supplied to the 
terminal Vr1 is then that which appears at the output of the amplifier 
IC5. 
It will be noted that the output terminal of the amplifier IC4 is fed from 
the mid point between two resistors of equal value (in this case 4.7K) 
which are connected between the 5v and 0v lines. Thus, this terminal is 
held at 2.5v. This is optimum in that it allows the voltage at the 
-terminal to swing widely either side of 2.5v without approaching too 
close to zero volts. Finally, for completeness mention is made of a number 
of the terminals of the circuit IC1 which have not already been referred 
to. The terminals Vcc and REG are associated with the power supply. Power 
is received at the terminal Vcc, and insofar as this differs from 5 volts 
a feedback signal is produced at the terminal REG which controls the 
voltage regulator in the sense required to bring the input voltage to 5 
volts. The terminal OSC is connected to ground via a capacitor the value 
of which determines the internal frequency of the circuit IC1 and 
frequency of the square wave appearing at the terminals .phi.1 and .phi.2. 
The inputs labelled CDSM have a capacitor connected across them, the value 
of which is determined by the internal frequency of the circuit IC1. The 
terminals R.sub.1 have a resistor connected across them the value of which 
provides a coarse gain control. The terminal R.sub.2 is connected to 
ground via circuitry which includes a variable resistor, the value of 
which can be adjusted to provide a fine gain control. In this context, 
gain control refers to the fact that the value displayed need not be the 
actual value of Vr1 divided by V.sub.in (that would be a gain of 1) but 
this value can be multiplied by a given factor (the gain value) to provide 
a displayed value which is more convenient to handle. The terminal Vro has 
applied to it a voltage of 1.25v, which is used for internal reference 
purposes. 
The invention has been particularly described above with reference to a 
glossmeter. However, as indicated at the outset, the invention is also 
applicable to other instruments. A hazemeter differs from a glossmeter in 
that whereas in a glossmeter the light detected by the detector 9 is that 
which has undergone specular reflection, in a hazemeter it is that which 
has been reflected by an angle which deviates by a specified angle from 
specular reflection. 
In a reflectometer, a surface of interest is illuminated at an angle which 
differs from 90.degree., and the light detected by the detector 9 is that 
which has been reflected at 90.degree. to the surface. A colourimeter is 
similar to a reflectometer, except that the sample is illuminated with 
light of a specific wavelength or it is illuminated with white light and 
the detector 9 detects only light of a specific wavelength. 
In an opacity meter, an object of interest is illuminated and the detector 
9 detects light scattered therefrom.