GB1560482A - Portable reflectometer - Google Patents

Abstract

Reflectometer for measuring the optical reflecting power of specimens by comparative measurement between the reflecting power of a reference object and the specimen to be measured. In order to keep low the supply of power to the light source required for this purpose, use is made of a pulse discharge tube (T) whose flash discharge is produced by the discharging of a capacitor (C2). A time-delay circuit leaves the digital display device which displays the measured values switched on only for a short, but adequate, read-off time. The power consumption of the reflectometer is reduced by the fact that a small storage battery suffices as voltage source, and the reflectometer therefore has a low weight, which enables it to be carried. <IMAGE>

Description

(54) PORTABLE REFLECTOMETER (71) We, URGES LIMITS, a company organised under the laws of Great Britain, of Unilever House, Blackfriars, London, E.c4, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to reflectometers, that is to instruments for measuring the optical reflectance of samples. Some such instruments measure the total amount of light reflected from a sample irrespective of colour, and others by use of filters are able to determine the relative percentages of particular wave lengths of the light reflected.Photometers, glossmeters, colorimeters and spectrophotometers are specific types of reflectometer to which the present invention relates.

The reflectometers currently in use are mainly designed for bench use, and to obtain consistent results the lamp forming the light source has to be on for lengthy periods. In view of this, such lamps are normally mains powered and, although battery power is sometimes used, this creates bulkiness for any instrument of reasonable accuracy and consistency.

The light source may be an incandescent electric lamp, a fluorescent tube or a high pressure xenon lamp. The type of tube is normally chosen for a particular application, and to have a spectral output to suit the particular application. However in each case the same requirement of keeping the lamp on for considerable time to provide consistence resulting in non-portability is inevitable in an accurate instrument.

iFor some reflectance measuring operations however, it is desirable that the instrument should be truly portable and the present invention aims to provide such an instrument.

We have realised that if an instrument comprises a pulsed discharged tube as the light source, and is operated in the multiple beam mode, i.e. with beams from the same source directed on to a reference and on to the sample to be measured, consistent results can be obtained with a small portable instrument Thus a portable instrument can be designed which is capable of being hand held, of being taken into any of a number of environments for doing market survey tests, of being used to obtain a reflectance reading from any article independent of the size of position of that article, and of being used to measure.

reflectance in awkward positions which a large laboratory device could not reach such as floors, walls, ceilings or even clothing which someone is wearing.

Thus, the invention provides a portable reflectometer having a discharge tube light source, a reference standard illuminated by the light source and means for simultaneously illuminating a sample to be measured from the light source as a single flash, reference standard measuring means and sample measuring means for measuring the light reflected off the reference standard and the sample respectively, and signal processing means for obtaining the ratio of the outputs of the two measuring means to determine the optical reflectance of the sample.

The pulsed discharge tube, generally a flash tube, is conveniently a capillary pulsed xenon tube having a quartz or glass envelope and preferably operates at a voltage of between 270 and 360 volts.

Preferably the outputs of each of the measuring means are stored, for example on a low loss capacitor, prior to comparison to determine the absolute value of optical reflect- ance of the sample. A delay switching circuit can be included for displaying the output of the signal processing means for a predetermined period.

With such an arrangement an output is obtained which is independent of variations in the absolute value of the output from the light source, and therefore the pulsed discharge tube becomes a practical and accurate instrument.

An embodiment of the invention will now be described by way of example with reference to the accompanying diagrammatic drawings in which: Figure 1 is a curve of spectral distribution of the light source; Figure 2 is a schematic diagram of the light source, its power supply and the beam reflectance; and Figure 3 is a circuit diagram of the signal processing means.

Figure 1 shows the spectral distribution of the light source used in the instrument according to the invention. The source is a pulsed xenon lamp and may have a glass or quartz envelope depending on the spectral response required, the divergence in distribution between quartz Q, and glass G being indicated on the curve.

The lamp used was one sold by Kendal Hype Associates with the type number XFS 47/360, slightly modified and having the characteristics listed below: Max. energy per flash - Watt secs, or joules 40 Min. firing energy mWs - milli-watt secs 1.5 Max. anode voltage. V 360 Min. anode voltage. V 270 Min. trigger voltage. KV 7 KV Flash duration at max. energy micro secs. 900 Average life to 10% drop in light o/p (Flashes) 2500 Arc length 30 mm Tube length 46 mm Tube diameter 4.5 mm The modification was to remove the ex ternal oxide trigger coating and replace this with a thin wire helix along the length of the tube on its exterior for triggering. This enables the correct spectral distribution particularly in the UV region to be achieved.

In Figure 2 the flash tube, T, its power supplies C2 and T.V. and light outputs B, and B2 and their detection are shown.

The flash from the tube is produced by discharge of an electrolytic capacitor C2 of value 16 UF after triggering from a high voltage impulse from a trigger voltage supply circuit T.V. The capacitor 2 is initially charged from a high voltage source H.V. at about 300 volts, and the trigger supply unit T.V. is a conventional arrangement producing several thousand volts and initiated by actuation of earthing switch PBI.

With this arrangement the light output is approximately 1 joule, which referring to the characteristic table, can be seen to be considerably under-running the lamp (maximum rating 40 joules) and thus increasing its life.

The lamp is located so that on flashing two beams B1 and B2 at right angles to one another are received on a sample port SP and a reference port aRP. The ports are located to receive the light at an incident angle of 45". Since the light emanates from the same light source at the same time, the proportion of light falling on each (approximately 1:1 dependent on the geometrical arrangement) remains substantially constant regardless of variations in the strength of light output.

For the usual measurements, the sample to be measured will be placed at the sample port, and a standard reference will be placed at the reference port The reflected light normal to the ports is collected by matching photocells MC1 and MC2. For application such as Glossmeters the cells are located to receive light at 450 and this angle can be varied for other applications.

For particular wavelength measurements matching filters F1 and F2 of selected colour may be placed in the reflected beams of light The outputs of the two photocells are then fed to a measuring and indicating circuit which is shown in Figure 3.

The two voltage pulses from the cell have an amplitude of vl and v2, volts with a pulse width of tl, and are amplified by two matched amplifiers A1 and A2, that give an output of nvl and nv2, n being the gain of each amplifier.

From the output of these amplifiers, the pulses are fed through two diodes D1 and D2 on to two capacitors Cl and {33, which charge up to the peak voltages nvl and nv2. This peak value is retained across the capacitors, so long as the input resistance of each following stage is high, and also the reverse resistance of diodes D1 and D2 is high.

In each case the following stage is a high input impedance buffer amplifier, BA1 and BA2 respectively. From the output of the two buffer amplifiers two steady voltages X and Y, are obtained with a time constant t2 seconds.

These are fed simultaneously into the inputs of a divider module, with an output function X 10 y which is then fed to a digital panel meter, DPM. With the reference set to give Y = 100 units and the sample = say 50 units, the output of the divider will be 50 10 X- = 5 100 units. This value can be scaled on the digital panel meter to read 50.0% and readings from zero to over 100% can be read directly. Due tJ the divider measuring a ratio of two voltages, X and Y, any common variation in these due to C2 and C3 discharging or the output pulses v1 and v2 altering due to a fall in light output is cancelled and the resultant output of the divider remains constant over a wide range.

To conserve as much power as possible when using the instrument, so enabling a small rechargeable battery pack to be used, a delay switching circuit DS was incorporated such that the digital panel meter DPM was only on for short periods. (Max 20 seconds). This delay period may be adjusted by potentiometer RV1 to ensure the operator lias time to record the reflectance reading. The delay circuit was initiated by a "press to read" push button (not shown) to operate a relay (not shown) which closes contact RLA/l to switch on the digital panel meter, whilst contacts RLA/2, RLA/3 remove the short circuit across capacitors C1 and C3 so enabling them to charge up to the peak values nvl and nv2.

After the delay period, relay RLA1 drops out and switches off the digital panel meter, whilst relays RLA2 and RLA8 discharge the capacitors C1 and Cl, so resetting them ready for the next reading.

With the total number of readings taken per day being of the order of 200, and each reading lasting up to 20 seconds, a total intermittent running time of 66 minutes per day was required. A rechargeable battery pack of 5 volts at 500 mA-Hour was chosen and found to be adequate for a day's running.

Each evening, after use, the instrument could be plunged into a battery charger to replenish the batteries for the next day.

WHAT WE CLAIM IS: 1. A portable reflectometer having a discharge tube light source, a reference standard illuminated by the light source and means for simultaneously illuminating a sample to be measured from the light source as a single flash, reference standard measuring means and sample measuring means for measuring the light reflected off the reference standard and the sample respectively, and signal processing means for obtaining the ratio of the outputs of the two measuring means to determine the optical reflectance of the sample.

2. A reflectometer according to Claim 1 having capacitors for storing signals proportional to the outputs of the two measuring means and a delay switching circuit for displaying the output of the signal processing means for a predetermined period.

3. A reflectometer according to Claim 1 or Claim 2 in which the discharge tube is a xenon tube.

4. A reflectometer according to claim 3 in which the xenon tube operates at a voltage of between 270 and 360 volts.

5. A reflectometer substantially as herein described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   the peak voltages nvl and nv2. This peak value is retained across the capacitors, so long as the input resistance of each following stage is high, and also the reverse resistance of diodes D1 and D2 is high.

   In each case the following stage is a high input impedance buffer amplifier, BA1 and BA2 respectively. From the output of the two buffer amplifiers two steady voltages X and Y, are obtained with a time constant t2 seconds.

   These are fed simultaneously into the inputs of a divider module, with an output function X 10 y which is then fed to a digital panel meter, DPM. With the reference set to give Y = 100 units and the sample = say 50 units, the output of the divider will be

   50 10 X- = 5

   100 units. This value can be scaled on the digital panel meter to read 50.0% and readings from zero to over 100% can be read directly. Due tJ the divider measuring a ratio of two voltages, X and Y, any common variation in these due to C2 and C3 discharging or the output pulses v1 and v2 altering due to a fall in light output is cancelled and the resultant output of the divider remains constant over a wide range.

   To conserve as much power as possible when using the instrument, so enabling a small rechargeable battery pack to be used, a delay switching circuit DS was incorporated such that the digital panel meter DPM was only on for short periods. (Max 20 seconds). This delay period may be adjusted by potentiometer RV1 to ensure the operator lias time to record the reflectance reading. The delay circuit was initiated by a "press to read" push button (not shown) to operate a relay (not shown) which closes contact RLA/l to switch on the digital panel meter, whilst contacts RLA/2, RLA/3 remove the short circuit across capacitors C1 and C3 so enabling them to charge up to the peak values nvl and nv2.

   After the delay period, relay RLA1 drops out and switches off the digital panel meter, whilst relays RLA2 and RLA8 discharge the capacitors C1 and Cl, so resetting them ready for the next reading.

   With the total number of readings taken per day being of the order of 200, and each reading lasting up to 20 seconds, a total intermittent running time of 66 minutes per day was required. A rechargeable battery pack of 5 volts at 500 mA-Hour was chosen and found to be adequate for a day's running.

   Each evening, after use, the instrument could be plunged into a battery charger to replenish the batteries for the next day.

   WHAT WE CLAIM IS: 1. A portable reflectometer having a discharge tube light source, a reference standard illuminated by the light source and means for simultaneously illuminating a sample to be measured from the light source as a single flash, reference standard measuring means and sample measuring means for measuring the light reflected off the reference standard and the sample respectively, and signal processing means for obtaining the ratio of the outputs of the two measuring means to determine the optical reflectance of the sample.
2. 2. A reflectometer according to Claim 1 having capacitors for storing signals proportional to the outputs of the two measuring means and a delay switching circuit for displaying the output of the signal processing means for a predetermined period.
3. 3. A reflectometer according to Claim 1 or Claim 2 in which the discharge tube is a xenon tube.
4. 4. A reflectometer according to claim 3 in which the xenon tube operates at a voltage of between 270 and 360 volts.
5. 5. A reflectometer substantially as herein described with reference to the accompanying drawings.