GB2229034A

GB2229034A - A light source

Info

Publication number: GB2229034A
Application number: GB8905010A
Authority: GB
Inventors: David Osborn Wharmby
Original assignee: Thorn EMI PLC
Current assignee: Thorn EMI PLC
Priority date: 1989-03-04
Filing date: 1989-03-04
Publication date: 1990-09-12
Also published as: GB8905010D0

Abstract

A light source comprises an incandescent lamp (2) having a filament (8) and means (28) for filtering radiation from the filament (8) to produce a plurality of narrow spectral lines. The lamp (2) is so configured that the temperature of the filament (8) changes rapidly in response to changes in the power supplied to the filament (8). In a method of simulating a discharge lamp for testing colorimetric instrumentation, the power supplied to the filament is periodically changed. A light source operated by this method simulates a discharge lamp in that the spectral power distribution (SPD) of the light source varies in time and with wavelength. <IMAGE>

Description

A LIGHT SOURCE This invention relates to a method of simulating a discharge lamp for testing colorimetric instrumentation, such as a spectroradiometer or a filter colorimeter, and to the provision of a light source for such a method.

In the manufacture of certain types of lamps, the colour properties of the lamps are assessed using a spectroradiometer.

The instrument provides an absolute or relative measurement of the spectral power distribution (SPD) of a light source, from which its colour co-ordinates x, y - a quantitative measure of the colour properties of the source - may be calculated.

Lamps not having the required colour specification, i.e.

having colour co-ordinates outside a certain range, typically within + 0.01 of specified values, are rejected. However, the accuracy required of a spectroradiometer to produce such values of colour co-ordinates is very high and can be close to its limits of measurement, such that a badly set-up spectroradiometer may produce measurements with errors of this magnitude. Accordingly, lamps which, in fact, have the required colour specification, could be rejected on the basis of erroneous measurements.

To the inventor's knowledge there is no known test source for checking the accuracy of a spectroradiometer. It is not possible to use an ordinary discharge lamp as a test source because of its relative lack of stability, i.e. repeatability of results. The colour co-ordinates of certain discharge lamps can vary over time in the range of from 0.003 to 0.02 in both x and y. Known incandescent lamps are more stable than discharge lamps - the colour co-ordinates of a good quality tungsten filament lamp can be repeatable to better than 0.001 in x and y over a period of time - but do not have their spectral and time-dependent characteristics.

The SPD of a discharge lamp varies enormously with wavelength including both sharp spectral lines and a continuous spectrum. Accordingly, incorrect spectral sampling may lead to either overestimation or underestimation of spectral power at certain wavelengths related to the sampling interval because of the very rapid change in the SPD in the vicinity of a spectral line. Moreover the SPD also varies (usually by a large amount) with time in the A.C. cycle of the supply. Incorrect temporal sampling can result in the SPD from part of the A.C. cycle being exaggerated relative to that in the other parts. Errors resulting from both incorrect spectral and incorrect temporal sampling can lead to very significant errors in the colour co-ordinates derived from the spectrum.

At present, the only effective way of testing the accuracy of spectroradiometers is to take part in an inter comparison exercise. In this, a laboratory (usually a National Standards Laboratory) circulates a number of e.g. discharge lamps to investigate whether or not spectroradiometers of participating laboratories produce the same answers for the measurement.

However, not only does this take time (of the order of years), but the relatively poor stability of lamps currently available produces rather inconclusive results as to the accuracy of the spectroradiometers.

It is an object of the present invention to provide a method of simulating a discharge lamp for testing colorimetric instrumentation and a light source for such a method which at least alleviates some of the problems outlined hereinbefore.

According to a first aspect of the present invention, there is provided a method of simulating a discharge lamp for testing colorimetric instrumentation including the step of providing a light source comprising an incandescent lamp having a filament and means for filtering radiation from said filament to produce a plurality of narrow spectral lines, the method further including the step of periodically changing the power supplied to the filament, wherein the lamp is so configured that the temperature of the filament changes rapidly in response to changes in the power supplied to the filament, whereby, in use, the light source simulates a discharge lamp in that the spectral power distribution (SPD) of the light source varies in time and with wavelength.

Such a method provides a reliable method of testing that colorimetric instrumentation such as a spectroradiometer can sample correctly all the spectral characteristics of a lamp, such as a discharge lamp, over a wide range of wavelengths and including variations in time. Such a method can be used on a daily basis to test the correct operation of a spectroradiometer and may also be used in the intercomparison exercises described hereinbefore to produce more conclusive results.

According to a second aspect of the present invention, there is provided an incandescent lamp suitable for use in the method of the present invention, the lamp comprising a filament of a low heat capacity and a fill of a gas having a high thermal heat conductivity, the incandescent lamp being so configured as to create, in use, a laminar flow of gas past the filament, whereby the filament can be cooled by said laminar flow.

As the filament can be cooled by the laminar flow of gas it can respond rapidly to a decrease in the power supplied by a decrease in temperature. Because of this the temperature of the filament can change rapidly in response to changes in the power supplied to the filament. Accordingly such a lamp can be used in the method of the present invention and is effective as a test source for colorimetric instrumentation.

An embodiment 6f the invention will now be described, by way of example only and, with reference to the accompanying drawings in which: Figure 1 shows a schematic view of a light source in accordance with the present invention; Figure 2 shows the time-averaged spectral power distribution of the light source of Figure 1.

As shown in Figure 1, the light source comprises an incandescent lamp 2 formed of a glass bulb 4 and an Edison screw cap 6. A lightweight, thin filament 8, made of a refractory metal, such as tungsten, and having a low heat capacity, is mounted on rigid lamp supports 10, 12 inside the bulb 4. The filament 8 may be a straight wire or a coil and provides a linear or point source of radiation. Preferably, the length of the source of radiation as presented to the spectroradiometer is less than 10% of the separation of the light source and the spectroradiometer, i.e. a length of less than 3cm to 5cm for a separation of 30cm to 50cm. The lamp supports 10, 12 provide a conduction path for heat away from the filament 8.Short tungsten coils (not shown) made of a heavier gauge wire than the filament may be provided to connect the filament 8 to the lamp supports 10, 12 to provide resistance against mechanical shock.

Alternatively, as shown, the filament 8 may be clamped directly onto the lamp supports 10, 12.

The bulb 4 contains a gas fill 14 of helium, chosen for its high thermal conductivity so as to provide substantial gas cooling of the filament 8 when energised. A chimney 16 (shown schematically in Figure 1) surrounds the filament 8 to direct the flow of gas around the filament 8. The filament 8 may be linear (as shown) in which case it may be mounted longitudinally with respect to the chimney 16.

The configuration of the bulb 4 and chimney 16, i.e. their shape and size, is such as to permit a strong, laminar flow of gas past the filament 8 to provide convection cooling of the filament 8. Such a configuration may be optimised by standard fluid flow modelling techniques using appropriate computer programs (such as 'FLUENT' a general purpose fluid-flow modelling package produced by CREARE R & D Inc. of Hamover, New Hampshire, 03755 USA). The bulb 4 may also include other forms of flow control such as a flow deflector 18, provided to produce a smooth turn round of the flow at the bottom of the lamp and to prevent flow of gas into the glass seal (not shown) regions of the bulb.

Accordingly the design of the incandescent lamp 2 is such that the temperature of the filament 8 changes rapidly in response to energisation of the filament and other changes in the power supplied to the filament 8. In particular, there will be a rapid response by change of filament temperature when the filament 8 is connected to an A.C. supply. For this, the heat capacity of the filament 8 is sufficiently low so that it cools sufficiently to produce an appreciable change in colour temperature (say 500K) between energisations but not so low that it is fragile.

For use as a test source for a spectroradiometer, the incandescent lamp 2 is connected, at inputs 20, 22, to a power supply, such as a 50Hz A.C. supply 24, so that the power supplied to the filament 8 changes periodically with time.

Means, such as a diode 26, which allows current to flow for only part of the A.C. cycle and in one direction only, may be provided to allow a longer time between consecutive energisations of the filament 8 and hence greater cooling of the filament after it has been heated. The incandescent lamp 2 could also be connected to an A.C. supply through a thyristor, or similar device, which may be triggered at alternate cycles, or even larger intervals to promote cooling of the filament 8.

The incandescent lamp 2 is used in combination with a Didymium glass filter. Didymium glass is a glass doped with neodymium and dysprosium oxide (available from various manufacturers) which is used, inter alia, as a standard wavelength calibration for spectrophotometers and has the long term stability needed for the present application. The filter may be provided separately, as shown in Figure 1 (reference 26) or sealed directly onto the glass bulb 4. Other means for absorbing or otherwise filtering radiation, such as Holmium oxide doped filters, or combinations of filters, having an absorption characteristic which exhibits a plurality of steep absorption edges over a wide range of wavelengths may be used.

A test source comprising an incandescent lamp 2 and a suitable filter 26 and connected to a 50Hz AC power supply produces a SPD with sharp absorption edges which vary in time because the specially designed filament 2 is energised at periodic intervals (e.g. 50 or 60Hz if energised directly from an A.C. mains supply). It accordingly simulates many features of a discharge lamp in its SPD and can be used as a test source for a spectroradiometer or other colorimetric instrumentation.

Figure 2 is an example of a time-averaged SPD produced using a test source with a Didymium glass filter. This SPD is similar to that of a super high pressure sodium discharge lamp but has the stability of the incandescent lamp 2.

In particular, such a test source can be used to test various key criteria in the use of a spectroradiometer, e.g.

1. Correct temporal sampling of the light output waveform.

As outlined hereinbefore, if the sampling of the spectroradiometer is not standard, e.g. incorrectly locked to the mains frequency, this may exaggerate some parts of the A.C.

cycle relative to others with resulting errors in colour co-ordinates. The present invention provides a light source whose spectral power distribution varies with time and accordingly such errors may be more easily detected.

2. Correct spectral sampling of the light output waveform.

Some spectral sampling techniques can give rise to substantial errors unless used with great care. Errors, which may arise through the inherent characteristics of the optical measurement gear of the spectroradiometer, are more easily detected when the spectrum of a test source has a well-defined structure with rapid change of spectral output with wavelength as provided in the present invention.

Many spectroradiometers are computer controlled - the computer being told the starting wavelength and then moving the wavelength at which the spectroradiometer is measuring during scanning of the spectrum of the lamp. The accuracy of the scanning is also an inherent characteristic of the spectroradiometer. Errors caused by inaccuracies in the computer control or in the scanning of the spectroradiometer are more easily detected using the light source of the present invention as this produces a well-defined reproducible spectrum having some of the features which make the SPD of discharge lamps difficult to measure.

3. Linearity and stray light. Because of the wide range of intensity produced over a wide range of wavelengths, the test source of the present invention also provides some test of linearity of measurement and stray light errors.

Modifications to the embodiment described and within the scope of the present invention will be apparent to those skilled in the art.

Claims

1. A method of simulating a discharge lamp for testing colorimetric instrumentation including the step of providing a light source comprising an incandescent lamp having a filament and means for filtering radiation from said filament to produce a plurality of narrow spectral lines, the method further including the step of periodically changing the power supplied to the filament, wherein the lamp is so configured that the temperature of the filament changes rapidly in response to changes in the power supplied to the filament, whereby, in use, the light source simulates a discharge lamp in that the spectral power distribution (SPD) of the light source varies in time and with wavelength.

2. A method according to Claim 1 wherein the step of periodically changing the power supplied to the filament comprises the step of connecting the filament directly to an A.C. power supply.

3. A method according to Claim 1 wherein the step of periodically changing the power supplied to the filament comprises the step of connecting the filament to an A.C. power supply via means to pass current for only part of the A.C. cycle.

4. A method according to Claim 3 wherein said means to pass current for only part of the A.C. cycle comprises means to pass current in one direction only.

5. An incandescent lamp suitable for use in the method of any one of Claims 1 to 4, the lamp comprising a filament of a low heat capacity and a fill of a gas having a high thermal conductivity, the incandescent lamp being so configured as to create, in use, a laminar flow of gas past the filament, whereby the filament may be cooled by said laminar flow.

6. An incandescent lamp according to Claim 5 wherein the filament is surrounded by a chimney.

7. An incandescent lamp according to Claim 6 wherein the filament is linear and is mounted longitudinally with respect to the chimney.

8. An incandescent lamp according to any one of Claims 5 to 7 wherein a flow detector is mounted in the base of the lamp.

9. An incandescent lamp according to any one of Claims 5 to 8 wherein the filament is made of tungsten.

10. An incandescent lamp according to any one of Claims 5 to 9 wherein the mounting of the filament permits conduction of heat away from the filament.

11. An incandescent lamp according to any one of Claims 5 to 10 wherein the filament is connected to rigid lamp supports by wire coils to provide resistance against mechanical shock.

12. An incandescent lamp according to any one of Claims 5 to 11 wherein the gas is helium.

13. An incandescent lamp suitable for use in the method of any one of Claims 1 to 4, substantially as hereinbefore described with reference to, and as illustrated in, Figure 1 of the accompanying drawings.

14. The combination of an incandescent lamp according to any one of Claims 5 to 13 and means for filtering radiation to produce a plurality of narrow spectral lines.

15. The combination according to Claim 14 wherein said means for filtering radiation is sealed to the lamp.

16. The combination according to Claim 14 wherein said means for filtering radiation is separate from the lamp.

17. The combination according to any one of Claims 14 to 16 wherein said means for filtering radiation comprises means for absorbing radiation having an absorption characteristic which exhibits a plurality of steep absorption edges.

18. The combination according to Claim 17 wherein said means for absorbing radiation comprises a Didymium glass filter.