Light output is commonly mistaken for the millicandela (mcd) rating of an LED. The mcd rating only measures brightness or luminous intensity (Iv) at a single point. When talking about total light output, we have to take into account two specifications. Not only do we need to know the mcd rating, but also the spatial radiation pattern (viewing angle) because the two are directly related.

LED's with narrow viewing angles have a higher mcd rating because when you focus more light into a smaller area, the brighter that area will be, and therefore the higher the mcd rating.

For example a 4000 mcd / 30 degree LED will have about the same light output as an 8000 mcd / 15 degree LED. Or a 3000 mcd / 30 degree will have about 3 times the light output of a 3000 mcd / 10 degree LED.

When converting incandescent bulbs to LED's in cars, it is best to have the widest angle LED's with the highest mcd rating for proper light output.

Notice in this picture the beam pattern is very tight and focused to one spot. This is a problem. The light needs to be dispersed evenly to make the dash light up correctly. Using the LED like this created hot spots. (Areas on the dash that are much brighter than the rest.)

Flattening or "shaving" the LED makes a nice clean dispersed pattern. Here's the beam pattern of the LED after it was shaved. Notice how smooth and wide the pattern is. This is what you want so your dash will be evenly lit.