Apogee's new DLI-500 meter is designed to measure photosynthetically active radiation (PAR) 400–700 nm and is accurate under all light sources. If you are looking for a low-cost option that is accurate under sunlight and some broadband light sources see our DLI-400 model. In addition to measuring PAR, the DLI-500 meter calculates daily light integral (DLI) and photoperiod. DLI refers to the total amount of PAR or extended PAR (ePAR) incident on a plane in a 24-hour period, expressed in units of moles per square meter per day (mol m^-2 d^-1). Photoperiod is the total amount of time in which PAR or ePAR is incident on a plane during a 24-hour period and is expressed in units of hours (h). Both DLI and photoperiod influence plant growth and development, and they are often measured in greenhouses and growth chambers to aid in light management and decision making.

The DLI-500 meter has two different screen modes: stored data and live view data. In the stored data screen, it shows the DLI measurement, photoperiod, and which day the data was collected (up to 99 days ago). The live view data screen displays a running average of PPFD from the last 2.5 seconds.

The DLI meters consist of a cast acrylic diffuser (filter), optical filter, photodiode, signal processing circuitry, and an LCD display mounted in an ASA plastic housing. DLI meters are designed for single or continuous measurements of PPFD or ePPFD; DLI; and photoperiod. The DLI meter uses a USB-C cable (included with the meter) to download the stored data as a CSV file to a computer.