Engineering Guide

DAYSIM is based on a series of models which are summarized below. Please refer to the manuscripts listed under publications especially to the IBPSA book chapter for more details.

Annual Illuminance Calculations

At the most fundamental level DAYSIM offers an efficient way to calculate the annual amount of daylight available in and around buildings. To do so DAYSIM combines a daylight coefficient approach with the Perez all weather sky model and the RADIANCE backward ray-tracer. The resulting time series of illuminances, luminances, radiances or irradiances at user defined sensors points can be used for a number of purposes:

  • to derive climate-based daylighting metrics
  • to calculate annual electric lighting use for different lighting controls based on available daylight 

Climate-based Daylighting Metrics

Over the past decade a new family of daylighting metrics to describe and evaluate daylight in spaces have been developed. These metrics summarize the daylight availability over the year and throughout a space. Two prominent daylighting metrics which are calculated by DAYSIM are Daylight Autonomy and  Useful Daylight Illuminance.Daylight Autonomy is now being a recommend metrics by the Illuminating Engineering Society of North America (IESNA).

Electric Lighting Use

DAYSIM uses an occupant behavior model called Lighswitch to  model called Lightswitch to predict  based on annual illuminance profiles and occupancy schedules how occupants in a spaces are going to manually operate electric lighting controls and shading systems (see below). The model thus predicts overall electric lighting energy use in a space. Daysim also outputs an Internal Gains schedule as can be used by energy simulation programs such as EnergyPlus and eQuest to conduct an integrated thermal lighting analysis of a space.

Dynamic Shading

DAYSIM can also model spaces with multiple dynamic shading systems such as venetian blinds, roller shades and electrochromic glazings.In spaces with dynamic shading systems DAYSIM automatically generates multiple annual illumiance profiles each with the shading system(s) in a static position throughout the year. In a post-processing step it then uses the Lightwitch model to predict in which state the shading systems is going to be. 

Glare Analysis

DAYSIM uses the daylight glare probability metric to predict discomfort glare from daylight for different viewpoint in a scene through the year. Similarly as for the annual iluminance profiles DAYSIM generates annual daylight glare probability profiles for different shading device settings that in a post-process are then used to predict the setting of a dynamic shading system throughout the year.

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