UV output of DE metal halide lamps

ekovalsky · 11/11/2006, 08:37 PM

As most know the DE metal halide lamps include a UV shield, which significantly decreases PAR. I was surprised when I compared the spectral output of the same bulb/ballast combo with and without the shield -- as an example click here for spectral plots of the Phoenix 250W DE 14K bulb with EVC electronic ballast, with and without shield, sourced from Sanjay's site.

Air itself is opaque to wavelengths below 200nm or so. Regular glass is opaque to wavelengths below 350nm or so -- indeed the lamp's measured output falls to nil around 350nm because of the glass envelope of the bulb. So the DE lamp UV output is limited to the UVA range of 350-400nm.

The shield appears to reduce UVA output by no more than 30%. Thinking in terms of sun block, this is basically SPF 1.3. Does this relatively minimal protection really make a difference to the animals in the tank ?

IndyReefMan · 11/12/2006, 02:28 PM

That doesn't sound like a lot, does it? But, there are other variables that we can control (eg- height ot the fixture above the tank and the shielding material that is used). Many manufacturers claim that their UV shields are UV treated. I'm not sure exacty what that entails but I suspect that they simply mean that the shield is tempered. Some materials, such as Lexan, can provide vastly superior protection to UVR, but until they are constructed in a way to withstand the intense heat of metal halide fixtures, they will have limited applications in our hobby. Here is a link to a very interesting article on this subject---

http://www.masla.com/reef/uvlighting.html

ekovalsky · 11/13/2006, 01:32 PM

Thanks for the link, it is indeed an interesting article.

Consider this -- the shield blocks only about a third of the UV light produced, and a 400w bulb will produce several times the amount of UV compared to a 150w bulb. So at least as much UV should be delivered to a tank lit by a shielded 400w bulb compared to a tank lit by a non-shielded 150w bulb. Yet obviously corals, fish, etc can and do thrive under multiple 400w bulbs.

Of course I have read about fish and corals getting burned after a shield cracks or breaks, or a new strong lamp is introduced. So obviously the UV is harmful. But this does make me wonder if the damage is caused by the sudden change in amount of UV to which the animals are exposed. In other words, can high levels of UV be tolerated if the animals are gradually acclimated to it ?

11/11/2006, 08:37 PM	#1
ekovalsky Premium Member Join Date: May 2003 Location: Scottsdale, AZ Posts: 335	UV output of DE metal halide lamps As most know the DE metal halide lamps include a UV shield, which significantly decreases PAR. I was surprised when I compared the spectral output of the same bulb/ballast combo with and without the shield -- as an example click here for spectral plots of the Phoenix 250W DE 14K bulb with EVC electronic ballast, with and without shield, sourced from Sanjay's site. Air itself is opaque to wavelengths below 200nm or so. Regular glass is opaque to wavelengths below 350nm or so -- indeed the lamp's measured output falls to nil around 350nm because of the glass envelope of the bulb. So the DE lamp UV output is limited to the UVA range of 350-400nm. The shield appears to reduce UVA output by no more than 30%. Thinking in terms of sun block, this is basically SPF 1.3. Does this relatively minimal protection really make a difference to the animals in the tank ?

11/12/2006, 02:28 PM	#2
IndyReefMan Registered Member Join Date: May 2005 Location: Indianapolis, IN Posts: 477	That doesn't sound like a lot, does it? But, there are other variables that we can control (eg- height ot the fixture above the tank and the shielding material that is used). Many manufacturers claim that their UV shields are UV treated. I'm not sure exacty what that entails but I suspect that they simply mean that the shield is tempered. Some materials, such as Lexan, can provide vastly superior protection to UVR, but until they are constructed in a way to withstand the intense heat of metal halide fixtures, they will have limited applications in our hobby. Here is a link to a very interesting article on this subject--- http://www.masla.com/reef/uvlighting.html

11/13/2006, 01:32 PM	#3
ekovalsky Premium Member Join Date: May 2003 Location: Scottsdale, AZ Posts: 335	Thanks for the link, it is indeed an interesting article. Consider this -- the shield blocks only about a third of the UV light produced, and a 400w bulb will produce several times the amount of UV compared to a 150w bulb. So at least as much UV should be delivered to a tank lit by a shielded 400w bulb compared to a tank lit by a non-shielded 150w bulb. Yet obviously corals, fish, etc can and do thrive under multiple 400w bulbs. Of course I have read about fish and corals getting burned after a shield cracks or breaks, or a new strong lamp is introduced. So obviously the UV is harmful. But this does make me wonder if the damage is caused by the sudden change in amount of UV to which the animals are exposed. In other words, can high levels of UV be tolerated if the animals are gradually acclimated to it ?