Do Gels lower a light's CRI?

[egproductions]

Wondering if geling a tungsten with a full CTB or geling a HMI with a full CTO effects the CRI of the light. Something I never thought about before but can imagine that perhaps it does.

 

[Moseph]

Interesting, but I wonder if it really matters at all. I would imagine that it could, since gels work by reducing certain wavelengths of light & don't actually make any new light or color, but I doubt that you're looking at a huge loss in CRI if it does happen at all.

I have observed that HMI's with full CTO tend to be much more green than real tungsten, hence why an HMI to tungsten correction gel exists. Of course this will still only get you an approximation since HMI lamps vary in color, but it does take out the typical amount of excess green that you'll get from full CTO.

 

[paulears]

CTO and CTB are intended for daylight to tungsten conversion. An HMI isn't really daylight, being a discharge source, so the output spectrum has gas spikes in it, which the correct gel tries to correct. CTO and CTB are designed for continuous spectrum. Lee have some nice videos http://www.leefilters.com/lighting/colour-list.html[video]http://www.leefilters.com/lighting/colour-list.html[/video]

They have useful graphs showing the curve.
Full CTO http://www.leefilters.com/lighting/colour-details.html#204&filter=tf
Full CTBhttp://www.leefilters.com/lighting/colour-details.html#201&filter=tf
HMI to Tungsten http://www.leefilters.com/lighting/colour-details.html#236&filter=tf

If you compare the last two you can see how different the response is.

 

[Andrew Luczenczyn]

Not to mention that throwing CTO on any daylight source is way less efficient than getting a true tungsten source.

 

[Moseph]

Not to mention that throwing CTO on any daylight source is way less efficient than getting a true tungsten source.

Not necessarily. There are plenty of reasons to use a real tungsten light as opposed to applying CTO to an HMI (annoying / impossible to nail the color, added weight of HMI's and having to deal with a large and sometimes exceedingly heavy ballast as well, added cost of HMI's) but if you can get over all of those things and you need pure punch on limited power consumption, a 1.2k HMI with full CTO should put out more light than a 2k.

 

[paulears]

My own rule of thumb is always that if you have to use correction, do it on the brightest source - and that includes windows, if you can wangle it. CT plus ND is good on windows with glass smaller than sheet size!