The IES defines illumination engineering terms in ANSI/IES LS-1-22, is available here online. You can search terms by their first letter using the categories listed in the Categories window, by the first letter of the term in the alphabetical list below, or by typing the term in the search window. You can find the Introduction material for LS-1-22 on these pages:
Copyright, Disclaimer, and American National Standards Statement
Foreword, Nomenclature Committee members, Introduction, and Scope

- ice detection light
- ideal radiator
- identification beacon (aeronautical lighting)
- ignitor
- illuminance
- illuminance (lux or footcandle) meter
- illumination
- incandescence
- incandescent filament lamp
- incident dose (exposure dose)
- incomplete diffusion (partial diffusion)
- index of illuminant metamerism
- index of observer metamerism
- index of sensation, M
- indirect actinic effect (of optical radiation)
- indirect component
- indirect lighting
- infrared lamp
- infrared radiation
- inhibition
- initial luminous exitance
- inky
- instant-start fluorescent lamp
- instrument
- instrument schedule
- integrated flash intensity
- integrating photometer
- integrating sphere
- intensity
- intensity (candlepower) distribution curve
- inter-lock
- interflectance method*
- internally illuminated exit sign
- interreflected component
- interreflection
- interrupted quick-flashing light
- inverse-square law
- iris
- irradiance (E)
- isocandela line
- isolation transformer
- isoluminance line
- isolux (isofootcandle) line