What Do I Need to Know to use Packaged LEDs for Communications and Sensing? 1. LED dice are generally 0.3mm to 0.5mm square, with a thickness of about 0.3mm. The smaller dice are, therefore, approx. cubic. 2. Most types emit from all facets. However, the base facet is usually metallized and therefore opaque. A few special purpose types have unusual metallization patterns (see below). 3. Since substantial amounts of energy emanate from the side facets, a 'die cup' serves to reflect such light in the same general direction as the light from the top facet, thus improving the forward power output or (for visible LEDs) luminosity. In the most popular bullet-shaped 'T1' and 'T1-3/4' plastic packages, the die cup is coined from the leadframe material, and it is necessarily relatively shallow. Silver plating improves optical and electrical characteristics. The fact that you can see the glowing die from the side of such packages is evidence that the cup is not entirely effective. The ultimate die cups are deep parabolic bowls with very smooth gold plating. 4. The bullet housing is an 'immersion lens' which works with the die cup to form the 'beam'. Owing to its directionality, the beam has much higher radiance or (for visible LEDs) brightness than a naked die. The immersion lens also provides a less abrupt transition from the index of refraction of the compound semiconductor material to the air, thereby reducing attendant losses. 5. When bullet LEDs are used in conjunction with objective optics, you have a choice about what sort of image you want to produce. You can: a)(approximately) focus on the immersion lens, or (b) (approx.) focus on an image of the die apparently located some distance behind the bullet package. Setup (a) gives you a fairly uniform 'condensed', round 'spot' at the focus distance, with a diameter equal to the bullet diameter times the magnification. Setup (b) gives you a square image of the glowing die with dark areas in the region of the wirebond and bond wire. This square will be surrounded by a 'halo' that is the image of the 45 degree angled die cup rim. The size of the square image will be approx. 7 times larger in each dimension than you would predict by multiplying the magnification of the objective by the size of the die. In other words, the immersion lens increases magnification by about 7x. This effect is easy to observe with your eyes. 6. You can also obtain many LEDs which substitute a flat top or window for the immersion lens. If you are trying to deliberately produce a small spot, this works best unless the dark zones resulting the wirebond and wire are a problem. Image size will be objective magnification times die size. 7. Leadframes are often poorly centered in the immersion lens package, making beam aiming with respect to the package a hit and miss affair. 8. LED mfgrs. Commonly offer several parts which use the same die type and lens shape but which vary in the axial location of the die cup relative to the top of the immersion lens. If you want to produce a round condensed spot using an auxiliary objective lens (see #5), roughly match this angle to the angle subtended by the objective lens at the chosen focal distance. 9. The most common top metallization pattern is a disc approx. 0.12mm diam. 0.025mm diam. Au wire is used to make the top contact. Bottom metallization covers the entire bottom facet, and attachment is either eutectic (solder) or by means of silver-filled epoxy. Given the internal junction structure of most high performance types and the interference of the metallization, higher radiance may be available from the side facets. Consequently, some special purpose parts are mounted on an insulating substrate and have ball bonds to each side of the junction, thus positioning an edge toward the 'top'. Hammamatsu have a part like this with a 'microball' lens positioned on the edge facet that produces spectacular radiance values. Much less exotic are fancier top metallization patterns such as Xs and grids which attempt to spread current over the junction at the expense of increasing the opaque area. 10. Compound semiconductor dice are tiny and brittle and the wirebonds are delicate. You can increase the peak output by driving them with short pulses, but you cannot pursue the tradeoff between amplitude and time past a certain point. Even for very low duty factors, most LEDs cannot handle more than 2Amps peak without greatly reduced reliability. Some are damaged by 500mA or less, regardless of pulse duration and freq. A few of the larger units double wirebonds can handle 10A pk. Roughly speaking, the visible types that have the amazing luminosity ratings are the most delicate. To put it another way, you can run most LEDs at their rated average current with much higher peak currents provided that peak current is an amp or less. Since most LEDs are rated about 40mA continuous, don't push the duration/amplitude tradeoff past about 4% duty factor without careful investigation.