I'm wondering though how you got such a high lux number with the calculator.
A 900mm reflector has a surface area of around 636,173mm2 minus the dead part in the middle where the LED heatsink is. You need to multiply this with the reflection losses. Since you are using a very old reflector I would go with 70% here. After this you just need to multiply this with the luminance of the LED.
COB LEDs have a rather low luminance. Your COB, the Cree CMT2890, has an LES of 22mm (diameter of light emitting surface). This allows for a rough calculation of the luminance by dividing the lumens by the surface_area x pi. At 15,000 lumens your LED should have an average luminance of around 12.6cd/mm2 (COB LEDs have many small dies with dead areas in between them so the real area is smaller and the actual luminance a bit higher). Multiplied with the area of the reflector I got around 8 Million Candela. This shows that you are leaving quite a bit of performance on the table. A Luminus SBT90.2 should easily reach 200cd/mm2 while giving you 1/3 of the lumens. Also, there a much brighter COB LEDs on the market if lumens are more important to you.
I would assume he’s using the 200cd/mm2 from the SBT90.2 otherwise it’s not above 100Mcd.
With 200 it does show ~120Mcd but that’s only with a perfect mirror, so you’re right he should be doing like 70-80% reflectivity to get more accurate numbers.
The variable for that is in the “intensity calculations” folder in the calculator
There is also a glass transmission variable which can be set to 1 since he is not using any front glass for the light.
You are correct I would trust your calculations over mine but when I looked again that number is for the sbt-90. My measurement is usually “it hurts my eyes so it must be bright” and “I pointed it at that house 2km away and they called the police it must throw a fair distance” haha around 3am one morning I lighted up someone sleeping on the job over 2km away.
I am trying different light engines out of curiosity and also to get a feel of what the mirror is capable of. SBT-90.2 is impressive, HID is good fun, CMT puts out a column of light. Looking at original light it was designed to be carried by hand so everything has handles built in, the genset was towed on a truck. By the looks of it the carbon arc assembly sits inside an enclosure inside the drum. Looks like it dropped in and was fastened so it didnt move. In my lights I would like to have it the same but build the cooling system, driver and maybe even a battery into the one module. That way if I was to hire them out depending on the use the light engine can be changed. I want one portable and one mounted on the mini dump truck. How would a plasma cutter or a tig welder go for a light engine? A small bottle of argon would last an hour or 2 to purge the oxygen from the electrodes about the same as carbon rods.
Screen shot from google of the 90cm carbon arc searchlight.
The things you are doing are very impressive and entertaining. I enjoy watching all your projects, of course the big mirror is top of my list. You sparked our interest in those WW2 search lights and the carbon arc movie theater projectors too. The search light mirror could shine 25 miles and the one you have did too. What history you are in possession is very interesting and honorable to give life too again.
