Newborn chicks raised in red light (
n = 16) became progressively myopic with a mean ± SD refractive error at 14 days of −1.62 ± 0.54 D that increased at 28 days (
n = 6) to a mean myopia of −2.83 ± 0.25 D (range, −2.50 to −3.25 D). Chicks raised in blue light (
n = 19) became progressively hyperopic with a mean refractive error at 14 days of +3.06 ± 0.29 D that increased to a mean hyperopia of +4.55 ± 0.21 D at 28 days (
n = 6; range, +4.00 to +4.75 D) (
Fig. 2). Even by 14 days the difference in the mean refraction of chicks raised in either red or blue light was highly significant (
P < 0.001). The red light–induced myopia and the blue light–induced hyperopia at 14 days were each significantly different to emmetropia (zero refractive error) (
P < 0.001). A small number of chicks (
n = 5) that were raised in white light (that had a slight excess of blue wavelengths), became mildly hyperopic at 14 days (mean +1.60 ± 0.25 D) (range, +1.25 to +2.00 D). On average, the vitreous chamber length of myopic chicks raised in red light for 14 days (5.55 ± 0.11 mm) was significantly longer than in hyperopic chicks raised in blue light for 14 days (4.73 ± 0.72 mm,
P < 0.001).
Chicks (
n = 6) made myopic by rearing in red light for 21 days (mean −2.21 ± 0.21 D; range, −2.00 to −2.50 D) rapidly became hyperopic (mean +2.50 ± 0.29 D; range, +2.00 to −3.50 D) when the lighting was changed to blue light for a further 21 days (
Fig. 3A), while chicks (
n = 6) made hyperopic by rearing in blue light for 21 days (mean +4.21 ± 0.19 D; range, +4.00 to +4.75 D) became myopic (mean −1.23 ± 0.12 D; range, −1.00 to −1.25 D) when lighting was changed to red light for a further 21 days (
Fig. 3B). The refractive error of chicks following blue light rearing for 21 days was significantly different to that following subsequent 21 days of red light rearing (
P < 0.001) as was the difference in refraction following 21 days of red light rearing compared with that following subsequent blue light rearing for 21 days (
P < 0.001).
During red light rearing followed by blue light rearing (
Fig. 4A) and blue light rearing followed by red light rearing (
Fig. 4B), body weight showed a linear increase with no change in the rate of growth when the chromaticity of rearing light was changed, and with a similar mean weight (and size) of chicks at the end of each period of rearing.
There was a gradual increase in mean ocular axial length during red light rearing, and this increase continued for the first 14 days of blue light rearing but ceased during the third week. The most notable finding was that the mean vitreous chamber length increased at a faster rate than mean ocular axial length during red light rearing, but decreased markedly and rapidly during subsequent blue light rearing (
Fig. 4A).
In blue light rearing followed by red light rearing (
Fig. 4B) a linear increase in mean body weight was not affected by either blue or subsequent red light rearing. The mean vitreous chamber length decreased between days 14 and 21 of blue light rearing as did the mean axial length, although to a lesser degree than the vitreous chamber length. During subsequent red light rearing, both the mean axial length and mean vitreous chamber length increased, the vitreous chamber length increasing at a faster rate than axial length during the first 2 weeks of red light rearing, but at approximately the same rate during the third week of red light rearing (
Fig. 4B).
Analysis of photographs of outdoor scenes in bright daylight conditions showed a preponderance of blue and green wavelengths and a deficit of longer red wavelengths (
Fig. 5). In contrast, tungsten light had a continuous emission spectrum that contained a preponderance of longer red wavelengths, a proportion of green wavelengths, but much less blue (
Fig. 6A). The spectral emission of a CFL, of a type that is replacing tungsten lamps for domestic use, revealed a discontinuous emission spectrum with a major peak in the red at 600 nm, a lesser peak in the green at 550 nm, a very much smaller peak in the blue at 430 nm, and virtually no UV emission (
Fig. 6B). In spite of the differences in spectral continuity, the emission spectrum of the tungsten lamp resembled that of the CFL both having a preponderance of red emission, a significant amount of green and a reduced amount of blue emission.