"K" or CRI

[ptonsparky]

Working on a project with a customer that uses different lamp types and colors to compare similar items. I don't have to design the system, I am just trying to get a better understanding of how they are doing it. The floresent lamps range from 2700K to 6500K along with a MH lamp thrown into the mix. Switching the lamps will give them an idea as to the quality of their product compared to a "perfect" test sample. The CRI on the MH is offered in a vary wide range depending on the lamp selected. The exact MH lamp information is hopefully coming before long. Is the "K" or CRI more important in this test?

 

[dicklaxt]

I think the K(Kelvin) value is the starting point temperature and would be the variable and the CR (Color Rendering) assigned value would be the resultant as indicated on the I(index)

That being said, to use a particular lamp for a particular task would vary depending on the comparisons being made and what they wanted to see under what condition.

I'll bet the same comparison could be found using different values ,,,,,,,,, ie: high temp and low index could at some point equal low temp and high index.SWAG

dick

 

[dicklaxt]

I first ran across these two terms a few years ago when I was doing or attempting to do some close up photography work and making a photo booth,never did figure that out or maybe it was the nut behind the camera.

dick

 

[PetrosA]

Basically, K refers to the color of the light, while CRI describes how accurately the light source allows the viewer to differentiate colors viewed in that light. Most HID lighting will make it impossible to accurately differentiate any colors at all. A pigment chart (vs. a printed one) is used to make the comparisons. The traditional standard for comparisons in the graphic arts, art and textile industries is 5000 K sunlight with 100 CRI but this changes when you are rating light sources with a different Kelvin rating. A fun way to see the effects of CRI is to take an assortment of paint chips with reds, greens blues and warm and cool browns and grays and look at them under various light sources. some lights will make the colors literally disappear.

 

[dicklaxt]

Close but no cigar,,,it is the color temperature of the light source



Definition of a Forum ,,,a place where people gather and agree to disagree

dick

 

[PetrosA]

Close but no cigar,,,it is the color temperature of the light source



Definition of a Forum ,,,a place where people gather and agree to disagree

dick

I'm not sure I understand the difference between what you and I wrote, but I don't see any need to argue. Kelvin values for light sources are often equivalents anyway, right? My monitor is definitely no where near 5000 degrees Kelvin or I'd be a lot tanner than I am. A halogen PAR lamp IS burning close to 2700-3000 K and you don't want to touch that. So, many K ratings are correlated rather than actual and this can be done by measuring the source (ex. monitor, LED, lamp) or the reflection of the source on a known color with calibrated equipment. The reflected temperature measurement is probably more useful for sources like LEDs since diodes of different colors are often grouped to adjust the temperature of the emitted light.

 

[dicklaxt]

Not arguing Peter just being the Devil's Advocate,my father once told me if we argue you will go away not learning,if we discuss you will be smarter for it.

have a good day my friend.

dick

 

[Electric-Light]

There's only such thing as real color temperature with incandescent lamps.

For discharge lamps, it is called correlated color which is the closest match to black body radiator color based on the chromaticity (expressed in x and y coordinates) of the lamp. It's like regression analysis.

you can have one light that's greenish and one that's pinkish and still have the same CCT "K".

CRI is more precisely called Ra8 color rendition and its an arbitrary measure. It is based on measured rendition of eight CIE standard color chips. It's an outdated system in my opinion.
It means Rendition average of 8 chips.

There's another arbitrary measure called Ra14 which is computed the same way but six additional color chips are used.

Many trichromatic lamps are designed to look good on Ra8 test, but almost all of them have a R9 rendition of close to zero which is the deep red color chip used in Ra14 test but not in Ra8 test.

So, a GE SPX30/Philips TL80/Sylvania830, etc are all 3,000K CCT with ~85 CRI, but has a R9 of about 0, while a 3,000K halogen has an R9 of 100, and Ra14 and Ra8 of about 100.

There's pleasing distortion (GE Reveal, meat display lamp), visually accurate (Colortone 50 etc), spectrally accurate (film or scientific use, like xenon high pressure of filitered 5000K incandescent).. what's good depends on application.

 

[Electric-Light]

I'm not sure I understand the difference between what you and I wrote, but I don't see any need to argue. Kelvin values for light sources are often equivalents anyway, right? My monitor is definitely no where near 5000 degrees Kelvin or I'd be a lot tanner than I am. A halogen PAR lamp IS burning close to 2700-3000 K and you don't want to touch that.

5000K CCT doesn't mean its got same spectral power distribution as 5,000K blackbody. It simply means that if compared to it, 5000K is the closest match.

Hence correlated. Using an instrument, you can get the chromaticity for your monitor. On a graph that already has the blackbody curve already drawn in, mark the dot for your monitor's chromaticity. The closest intersecting line is your correlated color temperature.

See http://en.wikipedia.org/wiki/File:PlanckianLocus.png

 

[Hv&Lv]

To get a true K temperature color one would need to heat something black and look at the colors as the object gets hotter. Kelvin used carbon blocks. Different things will produce a color similar to what Kelvin discovered, at a much lower temperature, but will not have a true CRI unless it burns something black
Incandescents have a color rendering index of 100, since the filament heated is black. I think candles have a CRI of 100 also
The MH will be somewhere around CRI 50-80, depending on the coatings.
These sodium vapor lights we install have the worst CRI, somewhere around 20-35. Everything looks the same color under these.

 

[PetrosA]

5000K CCT doesn't mean its got same spectral power distribution as 5,000K blackbody. It simply means that if compared to it, 5000K is the closest match.

Hence correlated. Using an instrument, you can get the chromaticity for your monitor. On a graph that already has the blackbody curve already drawn in, mark the dot for your monitor's chromaticity. The closest intersecting line is your correlated color temperature.

See http://en.wikipedia.org/wiki/File:PlanckianLocus.png

I'm sorry for being unclear with that comment. I do know that my monitor is correlated to a K temperature. In fact, I messed up - I have it calibrated to D65 rather than D50 now. I used to use D50 on my CRT years ago, but since I don't do graphic design as much anymore I set my LCD to D65 so that the internet and movies look better. The good thing is that this thread finally got me motivated to calibrate it again after way too long a break...

 

[PetrosA]

There's only such thing as real color temperature with incandescent lamps.

For discharge lamps, it is called correlated color which is the closest match to black body radiator color based on the chromaticity (expressed in x and y coordinates) of the lamp. It's like regression analysis.

you can have one light that's greenish and one that's pinkish and still have the same CCT "K".

CRI is more precisely called Ra8 color rendition and its an arbitrary measure. It is based on measured rendition of eight CIE standard color chips. It's an outdated system in my opinion.
It means Rendition average of 8 chips.

There's another arbitrary measure called Ra14 which is computed the same way but six additional color chips are used.

Many trichromatic lamps are designed to look good on Ra8 test, but almost all of them have a R9 rendition of close to zero which is the deep red color chip used in Ra14 test but not in Ra8 test.

So, a GE SPX30/Philips TL80/Sylvania830, etc are all 3,000K CCT with ~85 CRI, but has a R9 of about 0, while a 3,000K halogen has an R9 of 100, and Ra14 and Ra8 of about 100.

There's pleasing distortion (GE Reveal, meat display lamp), visually accurate (Colortone 50 etc), spectrally accurate (film or scientific use, like xenon high pressure of filitered 5000K incandescent).. what's good depends on application.

You point out some serious faults with the whole CRI system. I'm very interested in good color but I also know that most "consumers" either can't see the difference due to lack of visual training or just plain don't care. Till that changes, good color rendition in high efficacy light sources will remain outrageously expensive and will stay firmly rooted in the "specialty" category.

 

[rt66electric]

how do you calibrate a monitor????

how do you calibrate a monitor????

I'm sorry for being unclear with that comment. I do know that my monitor is correlated to a K temperature. In fact, I messed up - I have it calibrated to D65 rather than D50 now. I used to use D50 on my CRT years ago, but since I don't do graphic design as much anymore I set my LCD to D65 so that the internet and movies look better. The good thing is that this thread finally got me motivated to calibrate it again after way too long a break...

Ugh ?? how does a person calibrate a monitor????

 

[Hv&Lv]

Ugh ?? how does a person calibrate a monitor????

Start > Control Panel > Color Management > Advanced >Display Calibration (Windows 7)

Or simply type "Calibrate Display" in the Windows search bar

 

[PetrosA]

Ugh ?? how does a person calibrate a monitor????

You need software and a colorimeter (a device that can measure the color values on your screen). It works in one of two ways:

It can work directly with some monitors and write data directly to the Look Up Tables in the monitor (this is data used by your monitor to convert the RGB signals from the video card to the electrical values needed for output). This is useful with LCD monitors as it allows the backlight to be adjusted directly to a desired level in addition to adjusting the RGB levels, sort of like making the adjustments on screen through the monitor menu.

Or you can create a monitor profile which is used by the OS and/or software to send the video card adjusted RGB values to send to the monitor to achieve the desired result.

It generally measures the grayscale values and makes adjustments so that they are even from black through white with no color tint to them. Some software also measures other colors in the spectrum to make fine adjustments. Together with graphics programs, it allows designers and photographers to pretty closely match what they see on screen with what will be output from a printing press or other printer.

 

[Speedskater]

Ugh ?? how does a person calibrate a monitor????

You could go to one of the photography forums, where they discuss this endlessly.
http://forums.dpreview.com/forums/1004
http://forums.dpreview.com/forums/1003

Or just buy a color temperature meter and software program.

The home theater people also do the same.
http://www.avsforum.com/f/139/display-calibration

 

[Speedskater]

I'm not sure I understand the difference between what you and I wrote, but I don't see any need to argue. Kelvin values for light sources are often equivalents anyway, right? My monitor is definitely no where near 5000 degrees Kelvin or I'd be a lot tanner than I am. A halogen PAR lamp IS burning close to 2700-3000 K and you don't want to touch that. So, many K ratings are correlated rather than actual and this can be done by measuring the source (ex. monitor, LED, lamp) or the reflection of the source on a known color with calibrated equipment. The reflected temperature measurement is probably more useful for sources like LEDs since diodes of different colors are often grouped to adjust the temperature of the emitted light.

The light color temperature and brightness are two very different things.
The blue in the Northern sky on a clear day is maybe 6500K but you won't even get warm.