Image courtesy of SOHO/EIT consortium. SOHO is a project of international cooperation between ESA and NASA.

Discussion

This kind of an extensive study brings forward many kinds of test results. Some of the measured lamps are obviously meant for a different purpose than being a sole light source of a terrarium. Measurement results of these lamps need to be handled as a separate group. For example, the UV spectrum of so-called full spectrum lamps and actual UV lamps cannot be compared. The lamps belonging to the former group are meant to produce visible light with as natural and uniform spectrum as possible. Their UV radiation - if any - is merely a by-product of any fluorescent lamp. These types of lamps should never be considered as the only light source for an animal requiring UV light. Equally, it can be said that lamps designed for maximum UVB radiation are not directly comparable with regular UVB fluorescent lamps. It should also be borne in mind that the sunlight used as reference has been measured at Finnish latitude (at summer solstice, when the sun is at its highest point of elevation). The natural habitat of most terrarium animals is further to the south, where the radiation coming from the sun penetrates the atmosphere almost vertically and thus has to pass through a much thinner layer of ozone and air. This has a major effect on the shortest wavelengths of UV irradiation reaching the surface of the earth. In these measurements, the reference sun appears exceedingly weak, especially in the UV range, when compared to the situation in the natural habitat of many terrarium animals. Another issue that may affect the D3 Yield Index is the fact that the spectrophotometer’s sensitivity had to be reduced during the measurement of reference sunlight because the total radiation flux of the sun was higher than that of the lamps by a factor of over 200. This results in a situation in which the sensitivity of the sensor might be insufficient in the extreme low end of the spectrum. This may cause the spectrum curve to fall off too early. Measurement with a specialised UV meter might have obviated this problem, but that would have been impractical within the scope of this study.