Dichroics

[Alessandro Machi]

I use a Dichroic on a 1K spot from time to time.

 

How much "brightness" in terms of f-stops am I losing when I use a dichroic? (I would do a test right now by simply taking the dichroic on and off the light but unfortunately the Dichroic recently broke.)

[Phil Rhodes]

Hi,

 

What sort of dichroic filter? There's a few (hundred.)

 

If it's a tungsten to daylight correction, which is what I suspect you're talking about, you lose exactly the same amount as you would with any full blue or 80A filter; the only difference with a dichroic filter is that it doesn't gradually bleach out due to overheating.

 

Phil

[Laurent Andrieux]

A dichroïc filter occures less loss than a gel. T to Daylight loss is only one stop loss when full CTB gel is 2 stops loss

[Phil Rhodes]

Hi,

 

How is that possible? I can believe that the substrate that the filter is made on would be less absorbtive than the gel, but that's not going to double the output.

 

Phil

[John Pytlak RIP]

A dichroic filter will pass the desired wavelengths, and reflect the undesired wavelengths, and absorb relatively little light. So a well designed dichroic can more efficiently change the spectral qualitly of the light than a subtractive filter.

 

Dichroic filters are commonly used in additive printers, and as IR filters in projection systems.