Photographer Physical Lights add several Photometric and Artistic units that you would find in other softwares. Lights intensities is a broad subject that requires a bit of reading to grasps all intricacies of the physics behind visible wavelengths and the perceived brightness of light.
Here is a quick summary of the different available units in Photographer:
Power (Watts): Cycles default Energy of a light source, it is a radiometric unit that does not take into consideration how the human eye perceives light.
It is NOT the wattage information you find on lightbulbs packaging.
The Luminous Efficacy is hardcoded to 683 lm/W. More information about Luminous Efficacy right below.
Advanced Power (Watts): Similar to Power, but with exposed Luminous Efficacy. Luminous Efficacy defines the visible amount of light in lumen, generated by 1 Watt of energy.
The default of 683 lm/W is an industry-standard representing the efficacy of an ideal monochromatic light source.
Unlike Power, Advanced Power conserves energy based on the cone angle of the spotlight, i.e a small cone will focus the energy on a smaller area, making it brighter.
Lumen: Luminous flux, photometric unit assuming a maximum possible luminous efficacy of 683 lm/W.
It should be Normalized by Color Luminance in order to take into account the Luminous Efficiency Function that describes the perceived brightness based on the wavelength of a light source.
Best for Point lights as this unit does not take into account the angle of the light. Lightbulb packages values can be used as reference.
Candela: Luminous intensity (luminous power per unit solid angle), photometric unit assuming a maximum possible luminous efficacy of 683 lm/W.
It should be normalized by Color Luminance.
Best for Spot lights to maintain brighness when changing the Cone Angle.
For Area Lights:
Candela per Square Meter (Nits): Luminance unit which allows to maintain brighness of an area light when changing its size.
Best for TVs and Monitors that use Nits to describe their pixel brightness.
Lumen per Square Meter: While not a scientifically documented unit, this is often used for LED strip lights. It corresponds to the Emission intensity of the Area light surface and should not be mistaken for Illuminance in Lux.
Color Temperature (in Kelvin) describes the color of the light emitted by an incandescent black body radiator.
Color temperatures over 5000 K are called "cool colors", while lower color temperatures (2700–3000 K) are called "warm colors". As a reference, a clear sunlight at zenith has a color temperature close to 5600K and a clear sky has a color temperature around 9000K.
Theses values can easily be found on light packaging, but keep in mind that modern RGB LED panels are not limited to Color Temperatures.
In the Add Menu, you will find the regular Blender Lights as well as new Physical Lights.
They are identical assets.
The difference is that Physical Lights will trigger the Placement Modal from Photographer, and will use the default settings you have set in the Add-on Preferences.
This is a much easier way to add lights to your scene with your favorite base settings.