• Lenses with power will give different readings as they are moved about. The higher the power, the greater the variation. Diopter powers of +/-8 were found to give readings which varied by up to 10% as the lenses were moved about. This is caused by light from the source being positioned in various locations on the photocell - sometimes even partially missing the cell!

• Positive diopter powers tend to give higher readings, A +8 diopter lens gave an average reading about 10% too high. This probably results from the light being concentrated onto the photocell.

• Negative diopter powers tend to give lower readings. A -9 diopter lens  gave an average reading about 10% too low. The light beam is probably being expanded - even to the point that some of it misses the photocell.

• Very thick lenses (regardless of power) seem to give slightly higher readings than their thinner counterparts with the same power.

COMMENTS

BPI Photometers accurately assess the upper portion of the UVA band between 350 and 400nm. All known lens materials when treated with a UV absorber move the natural cut-off of UV light up to 400nm. No UV light is passed in the lower portions of the UV A or UV B range. (See chart above). The BPI Photometer can be expected to make an accurate assessment of UV transmission because of these characteristics of UV lenses and UV absorbers. When treated with a UV absorber the natural cut-off point is moved into the region of the UVA that the photometer reads best.

In an untreated CR-39 lens, the error of a photometer may be 10% of the UV transmission reading. This may seem like a large range of error, however it is insignificant because of the amount of UV that is being passed by the lens. The UV transmission is clearly in the ‘Danger’ zone and the actual percentage of transmission is immaterial.

After the lens has been treated with a UV absorber and the UV transmission reading of the photometer is 1, the same error factor applies. Now however, the error may be 10% of a 1% transmission reading. Again the error is insignificant. The lens is clearly in the ‘Safe’ zone. It can be seen that the accuracy of a BPI photometer improves as the lens being tested approaches 100% UV blocking.

BPI Photometers are sold with a calibration lens that is traceable back to the National Bureau of Standards. BPI Photometer users should regularly check the performance of their meters with this lens to ensure continuing accuracy. BPI also sells photometers that read lenses to 380nm instead of 400nm.