A light meter is a portable photoelectric device that measures the energy of the light and its brightness at a given point and area. The amount of light measured at a given place is called Luminance. It is measured in foot candles or lux, in metric SI system. A foot candle is the brightness of the light, one foot away from the source and lux is the total luminous flux over a defined area. One foot candle is roughly equivalent to ten lux. Another measuring unit, Lumens, is the amount of one foot candle falling on one square foot of area.  Lighting bulbs are measured in lumens, for example, a 60-Watt bulb would give out 700 lumens or 120 lux of light. Comparatively, the natural light has an energy content of about 9W per 1000 lumen.

 In other words, one lux is one lumen over one square meter area and one footcandle is one lumen per square foot.  According to a study, 30 to 50 footcandles  is sufficient for most home or office work, which would be about 300-500 lux. A bathroom may require a minimum of 80 lux and the area under the staircase or in the corridors about 40 lux, whereas on a clear day the outdoor light level can be 10,000 lux.

To find the size of a tubular skylight required, you need to know the luminance of the area, with the help of a light meter.  Compare the averaged resultant lux reading of the meter with the typical lux requirements of an area, which varies according to the type of work, as defined by the company. If the luminance falls below the required level, then look for a skylight, which has a lux level sufficient to fill up the deficient amount of light, considering that the intensity of light in a suntunnel varies with the time of year and on cloudy days, as the angle of sun changes. If the area is large then you may need more than one suntunnel. Generally, for optimum natural light, the daylight area should be minimum 10% of the floor area, which would be averaging about 30 lux.

A 14”/350mm diameter rigid and flexible tubular skylight would deliver daylight of 6500 lumens and 2100 lumens respectively, during summers. A 10”/250mm rigid suntunnel delivers about 220 lux of light level on a sunny day, which surpasses the light produced by a 150 Watt bulb. With such a variety available, you will only be able to make the right choice if you knew the amount of light available in the desired area, and a light meter helps to do just that.