D3 Yield Index
The percentage of UVB radiation from the total output has often been taken as an indicator of the ability of a lamp to maintain vitamin D3 photosynthesis in the animal skin. According to research made on human skin, the issue is not as simple as that. MacLaughlin et. al. (1982) showed that the action spectrum of vitamin D3 photosynthesis is extremely narrow (see fig. 1). Vitamin D3 production takes place mainly in the wavelength range 295-300 nm, while the UVB range is generally specified 280-320 nm. This difference is significant especially at the top of the range, where vitamin D3 production ceases, but photodestruction by longer wavelength radiation still continues. This makes the use of UVB irradiance figure by itself a poor indicator of the effectiveness of the incident light in photosynthesising vitamin D3 in animal skin.
Use of the D3 Yield Index as a basis for evaluation of UV lamps gives a possibility to compare very different types of lamps with consistent criteria. The D3 Yield Index turns the attributes of a spectrum curve, that are otherwise difficult to compare, into an easily manageable and understandable form even for a novice keeper; they indicate with an unambiguous value the efficiency of a lamp to promote photosynthesis of vitamin D3 and by that, indirectly ensure sufficient level of calcium metabolism.
Because the action spectrum for vitamin D3 photodestruction was not available, it was not possible to evaluate the effect of this process on the D3 Yield Index. More research is needed to resolve this. An in situ analysis of 7-DHC and its reaction products is also required to confirm their relation to the D3 Yield Index. *
It should also be noted that the formula for calculating the D3 Yield Index has not been tailored to compensate for power differences between lamps. Therefore, in case there are two otherwise identical lamps the one with higher nominal power receives a higher index figure, proportional to the power difference.
The D3 Yield Indices calculated on the basis of measurements of this study illustrate the fact that the percentage of UVB from total irradiance is not necessarily directly related to the capability of a lamp to promote vitamin D3 photosynthesis. Most clearly this can be seen in the case of Sylvania Reptistar; although 6 % of its radiation is UVB, its D3 Yield Index is only mediocre. When examining the spectrum of this lamp, it can be seen that its irradiance is almost zero in the range 290-300 nm, which is the most important range for the index.
Similarly misleading, but in the opposite direction, is the UVB proportion of Active UVHeat: only 1 % - yet its D3 Yield Index is approximately the same as for Sylvania Reptistar. In this case, the proportion of UVB from the total amount of radiation is extremely small, but its irradiance is very high. The fact that the total irradiance of Active UVHeat is many times higher, 1308 µW/cm2 as opposed to Reptistar’s 188 µW/cm2, makes the percentage comparison alone look insufficient.
Of all the lamps measured, the best contributor to vitamin D3 photosynthesis in skin is Zoo Med ReptiSun, with a D3 Yield Index of 439. This result can be considered especially noteworthy for the fact that the lamp in question is only a 14 W unit, while other units in the study have a nominal power of 30-40 W. The spectrum of ReptiSun begins very low in the UVB range and the spectrum curve rises steeply. In the most sensitive wavelength for vitamin D3 synthesis, 295 nm, for example, the radiation of ReptiSun is already 1.8 times stronger than that of the next best lamp.
The second best lamp, a Zoo Med ReptiSun unit that had been used for 10 months, does not show remarkable weakening of UVB radiation. While examining the full spectrum, it can be seen that the irradiance of the lamp has dropped constantly throughout the whole spectrum. This means that the UVB radiation of a lamp does not cease abruptly, as is sometimes suggested, but seems to get gradually weaker along with the visible light. On this basis a recommendation to replace the UV lamp twice a year due to alleged fading of UVB radiation is at least to some extent unwarranted. The conclusion is of course based only on one individual lamp; differences due to manufacturing tolerances etc. are entirely possible. A detailed research with a larger sample would be required to confirm this issue.
The next best lamp, Narva Reptilight, is also very efficient in promoting vitamin D3 synthesis with a D3 Yield Index of 284. Additionally, its spectrum is exceptionally beneficial; radiation at 315-335 nm range which potentially destroys vitamin D3 is very low.
Next comes a group of four lamps, the results for which are rather equal. Their D3 Yield Indices are already less than half of that of the best product. In this group, the case of Sylvania Reptistar is interesting. Even though its irradiance in UVB range is the third highest of all tested units, its D3 Yield Index is only modest. Its spectrum curve starts to rise only at about 300 nm; therefore, the main proportion of its radiation is concentrated in the UVB-2 and UVA ranges, in which strong irradiation is known to be detrimental to the production of vitamin D3.
Another sample that attracts attention in this group is Active UVHeat. The spectrum of this lamp is almost a pure line. In the wavelengths where radiation exists, it is very strong. In other areas radiation is at the same level as for the 14 W ReptiSun unit. There is only one single peak in the UVB-1 range, at 302-304 nm - this establishes the D3 Yield Index of this lamp at a reasonable level. However, its spectrum has another peak, over 10 times higher, at 313 nm and beyond that generally rather high irradiance values which may contribute to photodestruction of existing vitamin D3 in skin.
The rest of the measured lamps are disappointing in their D3 Yield Indices. Even the best (Rolf C. Hagen Repti Glo and Life-Glo) of these units have an index figure that is only 5 % of that of the ReptiSun unit. Lamps in this group cannot be recommended for use as UV lamps with a purpose to promote vitamin D3 photosynthesis in skin.
Both True-Light units also belong to this group, and apparently their purpose is not to be actual UV lamps. They are full spectrum lamps that presumably have not had the amount of UVB radiation as one of their main design criteria; instead they are intended to have as constant and natural spectrum in visible light as possible.
The Reptile Desert 7 % UVB, manufactured by Energy Savers Unlimited (ESU), turned out to be a disappointment. Despite its name, the unit only produces 0.3 % UVB radiation and its D3 Yield Index is only 0.5.
The sunlight that was measured as a reference is in its own league, and naturally the result would only get better if measured closer to the equator: the D3 Yield Index of the sun (1000) is over twice to that of the best of all the units tested. Due to the manifold radiation strength of the sun across the entire visible spectrum, its proportion of UVB is only 0.3 %.
When examining the results of the reference sunlight it has to be kept in mind that most reptiles kept in terrarium originate from areas far south from Finland. Therefore, the radiation of the sun measured in Finland is not an appropriate reference for determining adequate strength of UVB radiation, except for evaluating the light supply for domestic outdoor enclosures. Realistic reference can only be obtained by measuring the radiation of the sun in the natural habitat of the animal. To verify the results published here, and to establish a common baseline, it would be necessary to do an analogous measurement with compatible equipment near the equator.
However, the D3 Yield Index of the sun should not be considered as an absolute target figure since the irradiation measured here can only be achieved in a short time frame at noon in midsummer. Artificial lighting produces a uniform level of radiation during the time lamps are switched on. In the morning and evening, and during seasons other than summer, the radiation from sun penetrates the atmosphere in an inclined angle due to the lower elevation of the sun. Effectively thicker layer of ozone filters more ultraviolet light, and consequently the relative strength of UVB radiation of sun decreases rapidly. This is naturally equally applicable to all parts of the globe.
The sensitivity of the measurement device had to be reduced during the measurement due to the strong radiation of the sun in the visible range. This may have led to a premature cut-off at the very beginning of the spectrum that may affect the D3 Yield Index of the sun.
In conclusion, there are distinct differences in the suitability of different lamps to promote vitamin D3 photosynthesis in skin. In many cases, the percentage of UVB stated by the manufacturer is so close to measurement values that any differences are most likely caused by variations in calibration and spectral sensitivity of measurement equipment. However, in some cases larger discrepancies were found. For this reason, the UVB percentage given in the retail packages of products should be looked upon with appropriate reservations.
*Update: A new study has been published, that provides more information on this relation.