Light Plate Apparatus (LPA): a general purpose, customizable, user-friendly, open-source instrument for performing optogenetics and other photobiology experiments in batch and adherent cell culture systems. Each well of a the 24-well plate can be independently exposed to variable intensity, millisecond time-scale signals from one or two LEDs. For intuitive programming of the LPA, we have also developed Iris, a freely-available graphical web tool (See Software page). A LPA can be assembled in a single day from 3-D printed and commercially available parts using basic electronics assembly equipment for approximately $150.