The luminous flux, or light output, is defined as the total quantity of light emitted per second by a light source.
Luminous flux density, at a point on a surface, is defined as the "luminous flux per unit area".
In other words, luminous flux density is also known as the illuminance, or quantity of light on a surface, or....lighting level.
The measuring unit in both United Kingdom and America is known as foot-candles of light.
In Europe it is also known as meter candle light.
One foot-candle of light is the illumination of light that falls on one side that is one foot away from a lit one foot candle and exactly intersecting with the light.
Its abbreviated form is written as 1 Fc=1 Lm/ft.
Similarly, one-meter candlelight is the illumination of light that falls on a side that is one meter away from a lit one meter candle and exactly intersecting the light.
It is also called Lux i,e. the flux of light being received in each sq. meter is called the illumination of one lumen.
The unit of the lighting level is the "lux" (lx) where 1 lx = 1 lm/sqM
When area is described in square feet, the unit for the lighting level is the footcandle (fc): 1 fc = 1 lm/sqFt.
The relation between the fc and lux is: 1 fc = 10.76 lux.
Light meters come in different price ranges. It is important to ensure the one used measures LED illumination. This writer's preference is that the light meter 'eye' is not fixed to the unit itself. With the 'eye' on a wire, the user can hold it away from his/her body and thereby lessen the chance of a distorted reading. (Their white shirt or dark coat will affect the reading if the 'eye' is close.)
Measuring for ultraviolet (UV) light requires a specially calibrated Digital Light Meter. Examples are below.
Pet breeders find Digital UV Light Meters extremely helpful.