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White, Yellow, and LW UV Penlight

Please recommend a white, yellow, or LW UV penlight.

What is the required or standard brightness and coloured temperature of a white and yellow light?

I understand that the required wavelength for a LW UV light is 365nm. What is the required brightness of a LW UV light?

Thanks in advance

White light usually varies from 6500K (brightest blue-white) down to about 2700K for CFL “warm white”. You can get yellower/redder if you take an incandescent bulb and dim it. For color grading, the standard is somewhere between 6500K and 5500K, close to the color of noon northern daylight in South Africa, which is where color grading started with diamonds. You will just have to look to find a penlight whose color temperature is specified. There are LEDs with about 5500K and some that have warmer light.

As far as UV is concerned, you will find those lights fairly dim compared to the white lights. There is no particular “required brightness.” You just use a UV light to see if your stone fluoresces and that is observed in a dark environment, so the light does not have to be real bright. The stone glows brightly or dimly and that is what you take note of…the color of the glow and its intensity, too. Both LW and SW are useful for gem ID, Make sure not to look directly at the SW UV light, it can damage your retina. You can get UV lights specifically for gem ID, like the Raytech Versalume, but these days there are lots of LW penlights and some sanitizing UV lights that will work for SW, too.


Thank you for the response.

For observing the star effect or cat’s eye effect, is there a standard brightness for the penlight?

Here’s a scientist after my own heart, looking everywhere for standardization! However, I think you are going to find few manufacturers showing any brightness levels for their penlights and virtually nobody asking for a number for a cat’s eye’s intensity. The main thing is how sharp, straight and well defined the eye or star is and how visible it is under the usual lighting conditions…and how well-centered. If you hold the penlight in your teeth (LOL) or directly over your head, the eye or star should be in the middle of the stone and easily visible. This will depend on the structure of the fibers in the stone and the height and angle of the dome. It is typical for stars to have somewhat misshapen legs in natural material, so a really well defined and straight star is a plus. If you hold the stone out at a distance where you would observe a ring and see a distinct star, that’s all that is required, but the more distinct, well-formed and straight the star or eye, the better the valuation. As far as valuing the stone, you are stuck with comparisons in the market, which is the ultimate indicator of value anyway. I don’t see any notes on the type of eye or star in my Gemworld Guide…you could check other valuation software to see if they grade the star or eye…I don’t think so.

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All LW UV lamps use a source that emits a range of low wavelength (high energy) light but which is filtered to give you a small “window” at about 365 nm. These filters degrade over time so the intensity can vary appreciably over time and depend on the age of the lamp. A high intensity lamp is usefull if searching terrain for fluorescent minerals but usually a penlight one is OK for bench use as long as you protect your eyes from direct UV light.

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