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What all lights have Color temperature?


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#1 Mathew Collins

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Posted 20 July 2016 - 07:49 AM

Hi,

 

Color temperature is based on theoretical black body radiating light.

 

Fluorescent lamps don't have a color temperature, because they don't produce light by heating an element.

 

Tungsten lights have Color temperature.

 

What all other lights have color temperature? What about HMI lights?

 

Could someone  provide some insights?

 

-Mathew Collins


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#2 Jay Young

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Posted 20 July 2016 - 08:04 AM

What?

 

Colour Temperature: the temperature at which a black body would emit radiation of the same color as a given object.

 

Fluorescent lamps have a color temperature.

HMI's, Carbon Arcs, LED's, Trees, buildings, all gods children got color temperature.

 

Standard commercial Fluorescent lamps don't emit a full spectrum of light.  There are fluorescent lamps that are better, such as those manufactured by Kino Flow.

 

HMI's are daylight balanced (meaning somewhere in the realm of 5500K), but to be sure it's best to measure the lamp as bulbs can go weird with age.

 

LED's ARE a color, but phosphorescent light panels which use LED are a bit different.


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#3 Brian Drysdale

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Posted 20 July 2016 - 08:06 AM

It might be better to read this in more depth; this also covers fluorescent lights. HMIs are regarded as having a colour temperature in practice (even though not black body) because of the selection of the elements , although it does change with age.

 

https://en.wikipedia...lor_temperature

 

https://en.wikipedia...arc_iodide_lamp


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#4 Stuart Allman

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Posted 20 July 2016 - 11:45 AM

Matthew,

 

Video lights have what's called "Correlated Color Temperature" (i.e. CCT).  This often gets abbreviated/simplified by equipment vendors to "Color Temperature."  A black body radiator will have a specific spectrum and will appear a certain color to human vision.  The idea behind CCT is that it's the closest *visual* match to a black body radiator at a specific color temperature.  Even though the spectrum's don't match, the lights CCT and CT will *appear* pretty close when viewed side by side on a neutral surface.

 

The video light has to have color output that visually approximates, to a reasonably close degree, a black body radiator, otherwise CCT doesn't make sense.  For instance, trying to rate the CCT of a green LED doesn't make sense. 

 

This doesn't mean a video light and black body radiator share a similar spectrum or ability to render colors similarly.  It just means that when you shine the light on a neutral surface, like a white piece of paper, they'll appear similar.  This is really evident when you look at a cheap LED light.  Even though the light has a rated CCT, it might turn a person's skin green, whereas a higher quality LED light with the same CCT won't.

 

Hope that helps,

 

Stuart Allman

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#5 Mathew Collins

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Posted 20 July 2016 - 09:25 PM

Matthew,

 

Video lights have what's called "Correlated Color Temperature" (i.e. CCT).  This often gets abbreviated/simplified by equipment vendors to "Color Temperature."  A black body radiator will have a specific spectrum and will appear a certain color to human vision.  The idea behind CCT is that it's the closest *visual* match to a black body radiator at a specific color temperature.  Even though the spectrum's don't match, the lights CCT and CT will *appear* pretty close when viewed side by side on a neutral surface.

 

The video light has to have color output that visually approximates, to a reasonably close degree, a black body radiator, otherwise CCT doesn't make sense.  For instance, trying to rate the CCT of a green LED doesn't make sense. 

 

This doesn't mean a video light and black body radiator share a similar spectrum or ability to render colors similarly.  It just means that when you shine the light on a neutral surface, like a white piece of paper, they'll appear similar.  This is really evident when you look at a cheap LED light.  Even though the light has a rated CCT, it might turn a person's skin green, whereas a higher quality LED light with the same CCT won't.

 

Hope that helps,

 

Stuart Allman

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illuma.blogspot.com 

 

Thank you all.

 

Hi Stuart,

 

I knew the concept of color temperature

 

But i was reading this thread.

http://www.cinematog...ic=37864&page=2

 

When i red the posting by Chris Keth,

'Fluorescent lamps don't have a color temperature, because they don't produce light by heating an element.'.

 

That was a new information to me.  Tungsten Fresnel emits light on 'black body radiation'.

Apart from Tungsten is there any light fixture emits light by heating 'black body'?

 

 

.


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#6 Jay Young

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Posted 21 July 2016 - 07:19 AM

The definition of BlackBody: A blackbody refers to an opaque object that emits thermal radiation. A perfect blackbody is one that absorbs all incoming light and does not reflect any. At room temperature, such an object would appear to be perfectly black.

 

Lamps that emit light by thermal radiation caused by heating a black body... Incandescent are the only lamps I can think of that work in this way.  The spectral power distribution (“spectrum”) of light emitted from a blackbody is a function of its temperature only and is described by Planck’s radiation law.  That is to say, Color Temperature comes from the spectral radiation when a radiating source is held at a particular physical temperature.

 

LEDs emit light by a process called electroluminescence, not black body radiation.

 

All other fixtures either heat gas, or create an arc, but who's spectral color temperature is not based on the physical temperature of the medium.

 

If that were true, Arc lamps would have a color temperature of a lot.


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#7 Phil Rhodes

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Posted 21 July 2016 - 08:16 AM

Strictly speaking (and we appear to be speaking strictly) a tungsten filament isn't an ideal blackbody either, though it is quite close.


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#8 Stuart Allman

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Posted 21 July 2016 - 10:41 AM

Matthew,

 

This is all fun theoretical stuff for uber-nerd discussions, but really all that matters is that you're familiar with which CCT is appropriate for which scene in your project.  The ability of a light to have great color rendering is much more important than the underlying physics in my humble opinion.

 

Stuart Allman

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illuma.blogspot.com


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#9 Mathew Collins

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Posted 21 July 2016 - 01:00 PM

Thank you all.


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