Moisture Damage to Hoya Filter Glass
The filter glass that is used for shortwave and midrange ultraviolet light is manufactured by a company called Hoya in Japan. The product currently is the best known for blocking light waves in the visible spectrum, and at the same time allowing ultraviolet light waves to pass. The filter glass is very expensive, hence the high cost for shortwave and midrange ultraviolet lamps. It’s important to note that Hoya filter glass can actually be used for longwave ultraviolet lamps as well. Due to its high cost, most manufacturers of ultraviolet lamps tend to use filter glass manufactured by Kokomo for longwave lamps. Kokomo filter glass is much cheaper than Hoya, and yet still works very well in allowing longwave ultraviolet light waves to pass, and blocking visible light waves (however, Kokomo glass cannot be used with shortwave or midrange ultraviolet lamps). On ultraviolet lamps where there is both Hoya and Kokomo filter glass, the Hoya filter glass is very smooth in appearance, the Kokomo looks crinkled or almost cracked.
It is important to understand that Hoya filter glass is very sensitive to water. When this glass comes in contact with water, it actually absorbs the water and turns the glass to a white powder. This can happen quickly, after just a one-time exposure. This damage is permanent, and although you can remove the white powder, continuous exposure to water will permanently damage the filter so that it must be replaced. For people who are thinking about using ultraviolet lamps in moist or humid conditions, such as in mines or for water feature displays, care must be taken to prevent exposure of the filter glass to water. Even in areas with high humidity, such as Hawaii, this Hoya filter glass may become damaged. Way Too Cool has made ultraviolet lamps in the past with quartz glass over the Hoya filter, thus preventing moisture exposure.
Below are some pictures of ultraviolet lamps that have been exposed to too much moisture. Although ultraviolet light will still pass through these filters, you can see that the emission of ultraviolet light is being reduced by the amount of white powder. If you have an ultraviolet lamp in this condition, you can attempt to repair it by scraping off the white powder with a razor blade. You may notice that when the powder is removed, there is some pitting of the glass beneath. Once you have scraped off the white powder, you can use a glass polishing compound, like a stove top cleaner, to polish the filter glass. You can also use a Dremel tool with a buffing wheel to buff out the glass. This method will clean the filter glass, but will not repair the pitted areas. If the Hoya filter glass is damaged beyond repair, please contact Polman Minerals, and we’ll try to get you a piece of Hoya glass in the size you need.