Light colour temp

Sandtree

Anyone know of any cheap way of taking a roughly accurate colour temp of a light? Talking within 100K would be fine. 

Did try a couple of iOS apps but only found one that claims to measure colour temp without an accessory but for the single light it registered between 7600k and 4150k... I suspect the light isnt perfectly stable but its clearly not fluctuating between that range.

forgotmyname

It can vary depending on the surfaces around the light reflecting only part of the light back.

ie. a 7600k bulb in a room with a blue or red wall could alter the reading considerably.

To get an accurate reading you would need to make sure no light gets reflected from anywhere.

Does the camera hve a dedicated RGB sensor? Even with one it would need to be calibrated.

Sandtree

The readings using the app were taken using an iPhone 6s either pointing directly at the light source or at a "white" wall immediately next to the light and at 180 degrees from it. 

The app appears to give continuous readings but with a certain sensitivity, so minor camera move/shake don't change the rating but a material moves gets a re-evaluation. Tried with the phone on a tripod pointing at either source or white wall. Block the lens and uncover and the app re-evaluated with wildly different results. 

I want to get a 6500k output from a variable bulb, setting it to 6500k looks too blue compared to some other fixed 6500 bulbs but by eye its not possible to say which is wrong.

Heedtheadvice

Note possible sources of interfering errors above!

6500k is quite a blue lamp that most think of as very cold light. Even a supposed white surface reflecting the light will make it look different (which is why for measurement black body radiators are used), as will any other light source.

https://nofilmschool.com/2015/04/your-iphone-now-professional-light-color-meter-cine-meter-ii-update

Plus another thought. How is your colour vision? Is it Normal?

If not then the lamps that create a specific colour temperature by mixing the light sources inside the lamp may appear incorrect owing to non Normal sensitivity to various wavelengths. The lamp that emits a defined colour temperature can make up that equivalent colour temp from a different set of sources and thus look different to someone without good i.e. Normal colour vision. Both the lamps you mention can be correct (or of course be erroneous!) but you might perceive them differently.

Quite a lot of males are what was termed colour blind i.e. do not have Normal colour vision - in varying degrees.

Females do not have that problem as it is very rare for a female to not have Normal colour vision.

You might not even know unless you have taken a test.

Sandtree

I know I am red/green colourblind @Heedtheadvice so never the best one to judge these things... according to the Mrs who's played with colourblindness simulation apps she's green and champagne looks like sparkling swamp water to me. I'd say the two photos do look the same to me (she sees them as different) but wouldn't describe her as green. That said the link between linguistics and colour is an interesting topic.

That aside, I know 6500k is fairly cold/blue but its the notable difference of two solutions that both claim to be outputting the same colour temp in the same room makes it clear one of them isnt producing what it should be. Its not a major issue but just wanting to experiment with something and hence would like to know if the variable output light is outputting correctly or not as I can change it if the value is something other than 6500k at the moment.

CoastingHatbox

Hmm... you could probably do this with a dSLR and a grey card.

The grey card should be completely neutral. The bulb should be the only light source in the room.

The dSLR will have a means of setting the white balance, which will involve filling most of the frame with the grey card and pressing the shutter button.
The white balance selected by the camera should give you an indication.

Sandtree

Thanks @CoastingHatbox, hadn't thought of that... have a dSLR and some cheapo neutral cards (though they too look to have a colour caste to me but again that could be my eyes or the fact they were pennies and so potentially not accurate) 

wongataa

6500K isn't a specific colour. There is a variation in colours that have a colour temperature of 6500K.

D65 is a specific colour but You are unlikely to find light bulbs marked with that.

Sandtree

wongataa said:

6500K isn't a specific colour. There is a variation in colours that have a colour temperature of 6500K.

D65 is a specific colour but You are unlikely to find light bulbs marked with that.

D65 6500k is allegedly what I should want and therefore what I want to see if I can get with my existing variable bulb... if I can and I like it then there is a "calibrated" solution thats a bit pricy but claims to accurately and consistently give D65. Its not a product you can really test and then return (LED strips) if you decide you don't like it even if its what's "best"