help with evaluating different lights' quality, cri, white bal card in photoshop

R

rhodycam

Member
Hey Guys

New to videography, from photography and trying to better understand constant lighting. I just was wondering what the best way I have to check out different lights with a gray card and photoshop as my tools. I have been reading a lot on light temperature and cri. I do understand the concepts. However, it seems that most cri published values for lights/bulbs are questionable. I have a few lights I want to evaluate myself. I have read people discussing a light’s qualities by shooting a RAW image of a white balance card and examining it in photoshop. However, I cannot find any real discussion on how this is done step by step. The only one discussion I read was that you open the image in photoshop, select the white balance dropper tool on the gray card image and look at the specs in the right window, if the tint is +, the image is too green. So I am assuming a light that shows a low of a tint number is best. But I have also read general discussions about lights missing reds, etc..

Any help is appreciated
 
Sadly, what stills photography teaches you about lighting isn't too much help with cinematography.

I can (over) simplify it, but you know if I do that I'll leave stuff out and twist other stuff. You'll have to sort it out on your own. Oh well...

The sun is a continuous spectrum light. Unfortunately, there really aren't any artificial lights that can replicate sunlight. But approximating sunlight is generally the goal. Or I should say, replicating sunlight under certain conditions and times of year / day are the general goal, because sunlight is very much a variable, constantly changing from sunrise to sunset and beyond. And we aren't even talking about the effects of clouds...

The only widely used continuous spectrum light is tungsten. The ones used in cinema lighting run hotter then the ones used in home practical lights like reading lamps. The home bulbs run in the color temperature range below around 2700K, cinema lights hit around 3200K. Clearly, where tungsten lights are "deficient" if you call it that, is in the blue end of the spectrum. Certain tungsten halogen lamps can reach 5000K, but only in very small wattages, sufficient to light art on the wall but pretty much worthless for cinema.

The other class of cinema lights are gas-discharge lamps. This includes fluorescent, HMI (small versions of stadium lights), LEDs, etc. All gas-discharge lamps give a discontinuous spectrum -- the spectrum has odd peaks (commonly called spikes) and may entirely lack certain colors. There are a number of ways to combat this, but most play with the cost-efficiency-quality trio. IOW, I can give you high quality light, but it'll be more expensive and less efficient. Or I can give you cheap, but quality of light suffers. Etc.

The "sweet spots" for cinema lighting seem to have settled out to be fluorescent blubs with six or more phosphors (Kino Flo True Match and it's competitors), HMIs (a brand name that has become a generic description, same as Kleenex) from various sources, and LEDs which keep improving (the remote phosphor variant is showing some "legs" as it were). Oh yes, tungsten lights are still very much in use. Because it's hard to beat the skin tones you get from a tungsten light. Just sayin'.

What it comes down to is that all the lighting choices have deficiencies from a cinematography standpoint. And that's where white balance (aka color balance) comes in. Modern cameras have manual white balance capabilities. You bring up your lights, place a gray card (there are reasons to not use a white card, that's another discussion), and do a manual white balance. In modern cameras, this takes care of the most of the interesting peaks and valleys of a mix of lights. That is, it'll give you the correct color temperature (Kelvin number, the orange-blue axis), and it'll take care of the magenta-green axis too (the green spike from most fluorescent tubes), etc. Some cameras can handle a number of different color axes. Some cameras will give you access to manual adjustment of the axes (at least one of the Panasonic Varicam cameras as adjustment for 12 color axes, so you can match just about anything you have the skill and patience to match, but certainly so you can match a bunch of cameras together for broadcast).

So... no light is perfect, and the camera manufacturers have tried to help. But the bottom line is all you can really do is get close. Then when you images get to post production, you take it closer with a color correction step. Most professional productions will also use a step called color grading, usually after edit lock, and usually to aid in telling the story or for artistic intent.
 
Also - USE THE WISDOM HERE.

Once you've decided on the types of lights you think are the right tools for what you want to do - then ask folks here which exact makes and models give the best light quality at whatever price point you need to work with.

Types of lights would be; Fresnels, HMIs, Soft Fluorescents, LED light panels, LED light heads, etc.,
These are all very different kinds of lighting fixtures and it should be pretty easy to narrow down which you will need based on good online descriptions of what each does.
They all come in "Cheap", "Mid-Price" and "Premium" versions out there in the marketplace, and someone here has experience and details on ALL of them.

I think this will be the fastest way to zero right in on the best lights at whatever your budget is.
We've come to the point where every light out there has been pretty thoroughly tested and evaluated already so you should not have to re-invent the wheel here. Folks around here will give you the real low-down on any light you are looking at.
 
Just a quick technical comment, withoug having read the previous answers yet: you can't test CRI with a grey card. You need a bunch of colors, to see how each one reacts to the light. A colorchecker or spidercheckr should be enough. Link.

edit:
OK, now I have read the replies. There are two things here: white balance, and CRI. For white balance, a grey or white card will be enough. It tells you if the light has a color cast. It doesn't tell you anything about CRI. Because CRI relates to how different colors respond to the light, and can only be assessed by looking at the full spectrum of light, bit by bit. Or, as a second best, by looking to a few colors and seeing if they get illuminated or not by the light.
This manufacturer explains it reasonably OK.
 
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