When “color temperature” fails us.

A previous article talked a little about the confusion between color temperature and color “temperatures”  where cool temperatures are ‘warm colours’ and ‘cool colurs’ are warm.   Colour temperature is very useful to define the apparent colours of light.  Light colour can be accurately reproduced throughout the universe if its colour temperature is stated.

This is why your camera allows you to set your white balance by entering the colour temperature of your light source.  Your strobe manufacture probably states the colour temperature in °K right on the box, and incandescant lamps can be purchased by °K they will emit when operated at the correct voltage.

Before digital cameras, a photographer, knowing the colour temperature of the light source(s), could peruse the Kodak filter specification book and choose filters to alter the colours reaching the film to achieve his desired effect.

Color temperature is very useful when describing the colour qualities of the light emitted by something because it is heated to the point of emitting light.   However, it is completely unsatisfactory for describing light from Light Emitting Diodes (LED).  In my opinion, its use should cease immediately.

The problem is that ‘white’ LEDs are not white.  They do not contain ‘all colours in equal amounts.  Though they appear white to most people they are definitely not.  Rather than get into a lot of scientific explanations, it is very easy to prove this to your self.

Get a magnifying glass and look at your colour TV, colour computer monitor, etc.  Look closely and you will see there is no brown, magenta, pink, etc.  There is only red, green, and blue!  (OK, there is that one exception that we saw advertised on tv that actually added yellow emitters in addition to R, G, B.)

Now imagine that you used a selection of red lights and green lights to illuminate your photographic subject.  How do you think a blue dress would appear to the camera?  It would appear black.  A blue dress in sunlight appears blue because it reflects more blue light than other colours;  without blue light from your ‘flashlight’ there would be ‘no’ light to reflect, thus it would appear black.

Now, when we learn that ‘white’ LEDs actually work, we learn the problem.  “White” LEDs appear white-enough for most purposes because they use ultra-violet light to stimulate phosphor to glow.  This prinicipal, and the correct mixture of phosphorous compounds will generate light that looks ‘white’ to our eyes.  However, the light is missing quite a bit of red.

We also suffer from this same problem with the ‘new’, ‘efficient’, compact fluorescent lamps.
The top image shows the apparatus for this ‘home made’ spectrum scope.  The middle image shows an incandescant lamp on the left, and on the right shows a ‘rainbow’ of light which smoothly emits every visible colour in equal amounts.  The bottom image shows a “efficient” ‘compact fluorescent lamp’ on the left, and on the right, we can see that there is no orange, no yellow, no violet, no indigo, being emitted from this lamp.

The answer is called Colour Rendering Index (CRI)

This Wikipedia article explains CRI. and we will discuss it further in a future article.

Warmer, Colder, Kelvin, and Color Temperature

Please stop confusing me!

We all know the phrases “white hot”, “red hot”, “blue lips”, but these colors mean different things to different people.  That’s ok until we try to talk to each other.

What is “color temperature” ?

If you have experienced “bone chilling cold” you were certainly pleased when you could get inside in front of a fire or a ‘heat lamp.’   You felt more ‘warming’ than just getting inside the house with warm air.  Why?  Because the fire, or the old electric heater, is radiating vast quantities of infra-red light which penetrates through your skin and deeper into your muscles or joints.

If you have ever watched a man heat treat metal, you know the color of the hot metal is very important to them.  The color indicates the temperature.  The color you see being radiated is a function of the temperature.  The first colour you see is red; heat it more, and it will start glowing orange.  Keep heating it and if it does not melt, it will eventually get white hot.  This is exactly what colour temperature means.

Stating the colour temperature in degrees Kelvin means that your light source color is identical to something ‘perfectly black’ heated to that temperature.  Read much more about color temperature on Wikipedia.

Interior designers and photographers say they want ‘warm colours’ or ‘cool colours’ and what they mean is exactly the opposite of physics.  They want reds and oranges, “forgetting”, well, ignoring, that blue is hot, red is cold.  To me, the ‘coldest’ colour is white; the color of snow.

Will the Kelvin colour temperature actually tell us the colour?  I found a blog by Steve Jenkins  “Choosing the Right LED Bulbs” that may interest you.  (They are not BULBs even though thats what people are calling them.)  He has a chart that tells us the 2,000° K is ‘warm’ and 6,000° K is cool.

So please remember that if you are not educated in science that when you talk to someone who is, one of you is going to be quite confused.

Our next article will explain “When Color Temperature Fails Us”