Relative Light Units

pragmatic1

Could someone explain to me what relative light units are?

josey_whale

Presume you are talking about the output of a luminometer? If you are measuring the light output of say a chemiluminescence reaction, the results are usually given in RLU.

This is because luminometers typically do not yield a measurement directly in units of photons. The luminometer measures the current output by the photomultiplier tube, which cannot be translated into photons. So the light output of the instrument is relative to the current output - RLU.

In the same way, if you are measuring fluorescence, you will very often hear the term relative fluorescence.

pragmatic1

Thanks yeah its in relation to luminometry. However, a ccd camera is used rather than a photomultiplier tube. How is the current output relative to the light output? Thanks.

Tree

A CCD's output current is roughly proportional to the amount of light falling on it, in a similar manner to the photomultiplier tube. For the exact mechanisms behind current production in both devices you'll want to dust off your physics and research the devices themselves. From a biology point of view where the measured result is the important part, the mechanism itself isn't so important other than to know that PMTs are generally more sensitive to very low levels of light compared with CCDs and whether you choose a device with one over the other depends on the required levels of sensitivty.

pragmatic1

Tree wrote: »

A CCD's output current is roughly proportional to the amount of light falling on it, in a similar manner to the photomultiplier tube. For the exact mechanisms behind current production in both devices you'll want to dust off your physics and research the devices themselves. From a biology point of view where the measured result is the important part, the mechanism itself isn't so important other than to know that PMTs are generally more sensitive to very low levels of light compared with CCDs and whether you choose a device with one over the other depends on the required levels of sensitivty.

Cheers. Yeah was just wondering how it is proportional when photons dont have a charge. Its been a long time since I done instrumentation and google hasnt been much help.

Jimoslimos

pragmatic1 wrote: »

Cheers. Yeah was just wondering how it is proportional when photons dont have a charge. Its been a long time since I done instrumentation and google hasnt been much help.

Probably a good question for the physicists. It's been a while since I've done any physics but I think the simplistic answer is photon hits semiconductor layer and the energy causes movement of electrons.

I actually used to work with chemiluminescent assays for a number of years and never considered RLUs in any detail:o

Capt'n Midnight

pragmatic1 wrote: »

Thanks yeah its in relation to luminometry. However, a ccd camera is used rather than a photomultiplier tube. How is the current output relative to the light output? Thanks.

thought it would be optical depth but http://www.mgminstruments.com/FAQsTips.htm

RLU stands for "Relative Light Unit". This term is used because luminometers typically do not yield a measurement directly in units of photons.

so sounds like a fancy way of saying the output is linear but not calibrated into SI units

if you are using standards then not really an issue


The quantum efficiency - electrons per photon - depends on the detector material , wavelength and composition of filters