Photometric lights use data measured from real-world lights, often directly from bulb and enclosure manufacturers themselves. You can import IES profiles from companies such as Erco, Lamp, Osram and Philips; their IES files provides accurate intensity and spread data for a given light model.
Example IES lighting profiles
An example Maya file with IES profiles can be downloaded here (paths to the IES files will need to be edited).
Ai Photometric light found under the Arnold>Lights menu
Allows you to choose an IES light profile that defines the distribution of light.
The color of the light.
Intensity controls the brightness of light emitted by the light source by multiplying the color.
Certain IES light profiles can give noticeable noise due to high intensity hotspots. The obvious solution is to decrease the intensity of the light or move it away from nearby surfaces in order to reduce any brightly lit areas. If this is not possible, you can try decreasing the 'Indirect' attribute value of the light. Increasing 'Diffuse' samples will also reduce the noise (but will increase render times).
Many IES files originate from various lighting manufacturers, some of whom put extra text at the beginning of their IES files. If you are experiencing difficulties rendering a particular IES file, try opening the IES file in a text editor and deleting the header text from the top of the file. The file should typically start with
TILT=NONE and if it doesn't, that may be the culprit.