An LED has neither of these. First, it is an area source and light coming from that surface is not parallel. It would also be called a diffuse source, meaning light from all places on the surface travels in many directions. This kind of source can not be focused to anything but a smaller image of itself. The shorter the focal length of the lens, the smaller the image – but it is still an image of the source, not a spot. It is because of these rays, traveling in different directions, that a lens can’t focus them all to the same point. If you draw the side view of a lens and trace rays this all should be obvious.
Get the black “satellite grade” solar cell, such as Radio Shack 276-124 (version at least A). There are soldering surfaces on the top and bottom along one of the long edges. There may be oxide on those surfaces, so clean these surfaces very gently with very fine sandpaper or preferably fine steel wool groen laserpointer 200mw . Maybe place the cell on a flat surface while scrubbing it, since it is about as fragile as a piece of glass of the same dimensions. Then solder a pair of wires onto it, preferably 26-30 gauge. Solder quickly to avoid cooking the silicon.
Probably the best data I’ve seen that you can really see it but *certainly* in many cases it is stray shorter wavelength from diodes, we have measured it. For 1 W class sources a 10-9 level sideband can easily be the cause of the visibility groene ster laserpen 20mw , especially as the eye integrates up broad band featureless mess that spec. analyzers easily miss. Its easy to say definitely narrow band. but what is the bandwidth at the -80, -90dB level? For the Ti:S I guess you can be pretty sure though – I don’t recall how short the fluorescence can go.
I don’t know what the dynamic range of your spectrum analyzer is – and I’m sure the sidebands vary greatly from diode structure to structure. We have seen large wings on both sides of 780 to 810 nm diodes, sometimes very structured, sometimes broad and featureless. One 1.48 W diode was emitting astonishing amounts at 1.9 to 2 um for example. For a 1 W diode, say 10-9 or -90dB or 1 nW would be easily visible to the dark adapted eye and if it’s in the 600 nm-odd region (where we have seen emission) it’s that you will be seeing not the 1 W of 800-odd nm. The emission can be very broad, which your eye integrates up but an analyzer sees as a very flat signal just above noise; remember that for good dark adaption and narrow electrical bandwidths your eye is not *that* much worse than a PMT! Incidentally groene laserpen 50mW , since the photon has to cause photochemistry in the eye to get detected, I rather suspect that the drop in sensitivity with wavelength may well steepen. For example in my less careful youth I’ve looked at MW class 1.06 um lasers hitting things and never seen anything at all unless there is a plasma flash.
If you want a luminous efficacy figure in lumens per watt, many LED manufacturers publish such figures for the emitted light. If your LED does not have a figure for the efficacy of the emitted light, it is normally close enough to that of other LEDs of the same peak wavelength, bandwidth, and basic chemistry. Multiply the manufacturer’s figure by the LED’s conversion efficiency to get the overall luminous efficacy, in lumens of light per watt of electricity.