Download of DAYSIM 4.0.0 was on the developer's website when we last checked. We cannot confirm if there is a free download of this software available. The actual developer of the free software is Christoph Reinhart.
We cannot guarantee that the program is safe to download as it will be downloaded from the developer's website. Before launching the program, check it with any free antivirus software. DAYSIM is included in Photo & Graphics Tools. The most popular versions among DAYSIM users are 4.0, 3.1 and 2.1. You can install this free PC software on Windows XP/Vista/7/8/10/11 32-bit.
From the developer:
DAYSIM is a validated daylighting analysis software that calculates the annual daylight availability in arbitrary buildings based on the RADIANCE backward raytracer. It uses the Lightswitch occupant behavior model to mimic occupant use of personal controls such as light switches and venetian blinds and to predict energy savings from automated lighting controls such as occupancy sensors and photocell controlled diming systems.
You may want to check out more software, such as UrbanDaylight for Rhino, which might be similar to DAYSIM.