# # Light Intensity (PAR)

The PAR Sensor (Photosynthetically Active Radiation) is detecting and recording the ambient radiation between 400nm and 700nm in µmol photons × s⁻¹ × m⁻² using the light_intensity command (see previous chapter). But in contrast to the other sensors, the PAR sensor can also be used to control the light intensity output of all LEDs that have an emission peak within the PAR range. Three commands, light_intensity, previous_light_intensity and set_light_intensity are available to allow flexible control of the LED light intensity when using the PAR sensor.

## # Ambient Light Intensity

To replicate the ambient light intensity using the MultispeQ's LEDs, the ambient light intensity needs to be read from the PAR sensor and used as a setting for the intensity of the non pulsed lights.

To measure the ambient light intensity just add the light_intensity command to the protocols environmental array.

"environmental": [
[ "light_intensity" ],
...
],


Then the light intensity can be set for the non pulsed lights in the nonpulsed_lights_brightness array, using the light_intensity command.

"nonpulsed_lights": [
[ 2 ],
],
"nonpulsed_lights_brightness": [
[ "light_intensity" ], ...
],


This works for simple Protocols, as well as advanced Protocols using the _protocol_set_ command.

## # Previous Light Intensity

When using the light_intensity command to set the light intensity for a non pulsed light, with multiple protocols in a _protocol_set_, in each protocol the intensity also needs to measured, using the light_intensity command with the environmental array. In a longer running protocol, this would result in a fluctuating light intensity, especially when taking measurements outside. If the initially measured light intensity, is supposed to be used throughout the _protocol_set_, instead of light_intensity, previous_light_intensity can be used to set the non pulsed light intensity.


"_protocol_set_": [
{
// First Protocol in the set, measuring the light intensity
"environmental": [
[ "light_intensity" ],
...
],
"nonpulsed_lights": [
[ 2 ],
],
"nonpulsed_lights_brightness": [
[ "light_intensity" ], ...
],
},
{
// Second Protocol in the set, using the previous light intensity
"nonpulsed_lights": [
[ 2 ],
],
"nonpulsed_lights_brightness": [
[ "previous_light_intensity" ], ...
],
}
]


### # Light Intensity Update

The value for previous_light_intensity is updated whenever the following commands are used:

• light_intensity within the environmental array
• par_led_start_on_open, par_led_start_on_close and par_led_start_on_open_close with the Protocol flow control

Note

If the light intensity was never measured using the light_intensity command in the environment array, previous_light_intensity will return zero.

## # Set Light Intensity

Instead of using the light intensity reading from the PAR sensor, you can also manually set the light intensity using the command set_light_intensity. This becomes useful when a certain light intensity should be used rather then using the measured ambient. In this case, the commands for light_intensity and previous_light_intensity together with non pulsed lights will use the value defined in ser_light_intensity instead of the sensor reading.

In the following example, the light intensity is set to PAR value of 500 µE × s⁻¹ × m⁻².

"set_light_intensity": 500,   //(requires one integer, not a float)
"nonpulsed_lights": [
[ 2 ],
],
"nonpulsed_lights_brightness": [
[ "light_intensity" ], ...
],