Inconsistent colour temperature ?!??

[Kenneth Lau]

While ago I was shooting in an underground parking lot location where there was a lot of sodium vapour lamp. I wanted to get rid of those lamps since it was really ugly and I was told that I will not be able to gel it off because those lights have inconsistent colour temperature.

I ended up black wrapping every single lights and it worked. However, I still didn't understand what it meant by the colour temperature is inconsistent.

Does it mean that every lamp has different colour temperature? Or it means the colour temperature keep changing within a single lamp?

Can anyone explain this to me?

Thanks !

 

[paulears]

The colour temperature is NOT inconsistent the problem is that you are treating them like 'normal' lighting, that has energy in all the available wavelengths - so there is a bit of blue, some red, some amber, some green - bits of every shade. Tungsten lamps have more at the red end, others, badged as 'daylight' have more of the output energy in the blue end. You can use gel that removes a little red, to blue it up, or you can use another that removes blue to warm it up - you can even use another to try to remove the greenish tint that fluorescent often has. These work because all these types have some output from red to blue. They throw away quite a lot of light, of course, because they work by subtracting light at wavelengths you don't want. leaving a balanced result that approaches the desired colour temperature. Some discharge lamps use more unusual gasses - and can produce a lot of light - BUT, and it's a big but - these gasses produce most output in a very specific wavelength. In the case of neon gas, it's a rather nice red, mercury lamps are a blue-green colour, and loads of other colour come from using different gasses. Sodium produces a nasty nicotine colour, although you can get a sodium version with other gases as a mice that have a warmer colour. The big problem is that these are not continuous spectrum lamps - they produce colour at just a few very price wavelengths - so gel has no effect, remove the yellow, and there is no red, or blue remaining. This spike of colour also messes up camera auto white balance circuits, because they expect continuous (if a bit uneven) output. Some lamps even have spikes at infra red and UV - the UV one being very important - one reason why HMIs have an interlock stopping you from looking inside when its working.

So your 'inconsistent' word is really used in the wrong context - it's continuous, or non-continuous spectrum. I can see where people might have got the inconsistent term from, but that's wrong - discharge lamps are very consistent! Just not very video friendly.

 

[j1clark@ucsd.edu]

While ago I was shooting in an underground parking lot location where there was a lot of sodium vapour lamp. I wanted to get rid of those lamps since it was really ugly and I was told that I will not be able to gel it off because those lights have inconsistent colour temperature.

I ended up black wrapping every single lights and it worked. However, I still didn't understand what it meant by the colour temperature is inconsistent.

Does it mean that every lamp has different colour temperature? Or it means the colour temperature keep changing within a single lamp?

Can anyone explain this to me?

Thanks !

Sodium Vapor lights usually give off a dominant narrow spectrum of 'yellow' light. That means there are essentially no other 'colors' in the spectrum. A Filter works on the principle of allowing certain colors 'through', but if the light bulb doesn't give off any energy in a color... there's nothing to allow through.

This is why putting a 'blue' filter on a Sodium Vapor lamp will not work... there's no (or very little) blue in the spectrum. Tungsten light has a broad spectrum which includes some blue/green, hence one can 'cool' off a Tungsten light with a 'blue' filter... however, the cost is less light...

Technically there are two typical types of Sodium Vapor, low pressure which gives of a 'really yellow' light, and high pressure which has more residual spectrum outside of 'yellow', but essentially the only dominant color is yellow.

This is why things that are not yellow look 'dark' or oddly colored when seen under them.

 

[Greg Smith]

Another way to put it is that the physical concept of "color temperature" doesn't even apply to non-continuous sources like sodium vapor, neon, and even many fluorescent tubes that use gas discharges at discrete wavelengths to generate light.

Color temperature refers to objects that act as blackbody radiators -- that is, they are basically masses (candle flames, metal filaments, carbon arcs, or that big ball of hydrogen in the sky) heated up to a specific temperature, and the distribution of radiation from such an object is smooth, continuous and predictable if you know the temperature of the radiator. (Think of a blacksmith heating up a piece of iron. It goes from deep red, to brighter red, to yellow, and finally to white and even a bluish white as it gets hotter.) Lighting that derives from electric stimulation of a gaseous medium, or direct conversion of electricity to light as in a laser or LED, doesn't follow the assumptions of the blackbody model and, therefore, can't technically be said to have a "color temperature" per se. Fluorescent tubes and LEDs that have a relatively wide spectrum achieve this through phosphor coatings that absorb some of the light energy and re-radiate it at different wavelengths, and/or through mixing different colors of radiators in the same fixture (common with LEDs). They can achieve more or less acceptable color rendering for photography depending on how well this is done, but the spectrum of light they produce can look awfully ragged when viewed on a graph, not the smooth normal-curve kind of distribution you see from a tungsten filament or the sun. In almost every case, these tricks for extending and smoothing the color spectrum of such non-traditional light sources has the side effect of decreasing their luminous efficiency, so there is a tradeoff between color rendering, light output, heat and power consumption. Competing designs and lighting technologies emphasize these properties differently.

In the case of sodium vapor, the goal is to produce the maximum level of illumination in utility lighting applications for direct viewing by the human eye, while consuming the smallest possible amount of electrical energy. Color accuracy doesn't figure into the design in any way, and in fact they are basically monochrome sources that don't render much color at all in the subjects they illuminate. Filtering them can, basically, only make them dimmer, but not more colorful.

- Greg

 

[Bruce Watson]

...I was told that I will not be able to gel it off because those lights have inconsistent colour temperature.

Yes, and no. You can't "gel them off' but it's not because they have inconsistent color temperature. It's because they aren't full spectrum lights -- far from it. They put out a single spike that's a nice ugly amber. You can't supress that spike down to the level of the rest of the specturm, because the lamp isn't producing the rest of the spectrum. If you try you'll find that all you can do is make them darker. You can't get red and blue from a lamp that isn't producing any red or blue, no matter how you gel it.

So you did the right thing to block them completely, and use your own lighting.

 

[j1clark@ucsd.edu]

In the case of sodium vapor, the goal is to produce the maximum level of illumination in utility lighting applications for direct viewing by the human eye, while consuming the smallest possible amount of electrical energy. Color accuracy doesn't figure into the design in any way, and in fact they are basically monochrome sources that don't render much color at all in the subjects they illuminate. Filtering them can, basically, only make them dimmer, but not more colorful.

- Greg

Here in So Cal, in San Diego in particular, Low Pressure Sodium was required for years to protect the 'skylight' as seen by Mt Palomar (I think Mt Wilson people got LA to do essentially the same...) observatory. With the narrow spectrum of Low Pressure Sodium, the images take by the telescope could be 'filtered' for the city lights.

Things have changed, some areas are now converting over to High Pressure Sodium which has a broader spectrum, and Mt. Wilson and Palomar are doing less 'deep space' observation, which require lower 'dark sky' intensities...

 

[Kenneth Lau]

Thank you so much for the clarification guys. Greatly appreciated ! !