Zebra IRE question

[Tanner Stauss]

This question was in the Cinematography section and didn't get answered. I would like to see if someone can answer it here.

I've looked around for this question, but here it is.

I am used to zebra in % where 100 % is overexposed (no information captured).

In RED the zebra is presented in IRE. What is IRE, and how do I compare it or calculate % to the appropriate zebra IRE value?

Thanks

 

[Sidney L. Plaut]

IRE is the reference as to what is 100% or 0% -- I.E 100ire= 100 %.

the simple explanation : IRE is a scale used to measure brightness in electronic signals - I am sure some one more technically gifted can elaborate on this...

hope this helps - translate % to IRE - 50% = 50IRE

 

[Tanner Stauss]

Thanks Sidney. So if that is the case, why does the IRE setting allow you to go up to 108 IRE?

Wouldn't 108% brightness start melting the Mysterium?

 

[Michael Morlan]

IRE (Institute of Radio Engineers) is a unit of measurement of composite video and is a somewhat dated term in the digital era - until one views an image in the analog, SD, composite domain again. It is a bit arbitrary, where:

140IRE = white = 1v peak-to-peak
7.5IRE = black (with setup)
0IRE = super black
-40IRE = horizontal sync pulse/blanking level

The reason for the black with setup and super black was to allow luminance keying. That method was outdated in the 60's but we still struggle with it today, being required to submit video for broadcast with the blacks at 7.5 IRE. (Not sure about the tech requirements of REC709.)

Learn all about the wonderfullness that is the composite video signal here:

http://www.maxim-ic.com/appnotes.cfm/an_pk/734

The RED One measures digital illumination from 0 - 108. Again, an arbitrary measurement. I remember cameras being measured with 100 as clipping. Then some clever manufacturer showed how their camera could retain detail beyond 100. (Someone check me on how this came about.)

I most often set my high zebras from 103-106. That way, I can tell if a highlight is approaching clip by the displayed zebras. When the zebra disappears in the hot spot of a highlight (107-108) I know I am clipping.

Michael

 

[Tanner Stauss]

Good info, thanks Michael.