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Abstract
1. Previous investigations of the effects of light colour on the productivity or behaviour of chickens have not equated the intensity of the different coloured lights.
2. Ten pullets (Warren Studler 128) were used to determine the perceived intensity of two colours at opposite ends of the visible spectrum (blue, peak wavelength 415 nm and red, peak wavelength 635 nm).
3. Initially half of the birds were trained with food rewards to detect the brighter of two white lights and half the dimmer one.
4. Two discrimination tests then investigated the generalisation of this learnt ability (to distinguish white lights of different intensity) to coloured lights. They were rewarded for distinguishing the brightness of red and blue filtered lights when blue intensity was initially high and gradually reduced, and when red intensity was initially low and gradually increased. The birds identified the lights as being of equal intensity when the blue light was 3.6 times as bright as the red.
5. A second test examined the situation in reverse. A bright red light that was reduced was compared to a dim blue light that was increased in intensity. The birds were again successful and identified the lights as of equal intensity when the blue was 3.1 times as bright as the red. A final test confirmed that there was no difference in the perceived intensity of blue and red lights in the ratios 3.6 or 3.1:1.
6. The difference in perceived red and blue light intensities was less than predicted from the spectral sensitivity curve for chickens. It was expected that the blue light would be perceived as dimmer than the red because the red filter transmitted more light than the blue, and the tungsten filament light illuminating the filter emitted more long than short wavelength light. The observed discrepancy emphasises that perceived intensity is difficult to predict from spectroscopic measurements.
7. The birds then underwent a simple visual test, to discriminate the length of two lines, equally well in red or blue filtered lights that were equiluminescent as determined previously.
8. It is concluded that coloured lights can be equated for intensity by psychophysical testing, and that there was no difference in visual acuity under equiluminescent blue and red filtered lights.
