While purchasing a flashlight or a headlamp one has to consider the lumens instead of Watts. The amount of lumen quoted by the manufacturer should not be more than the lumens the product can actually provide. The total amount of light delivered by the product is measured in lumens. The intensity of the light produced by the product is measured in lux. Hence one should not confuse with lumen and lux. Lumen is just the total quantity of light delivered and is not related to quality of the light whereas lux indicates the quality of light delivered by the product. When we buy a headlamp or a flashlight from the Lumens we find out the total quantity of light the product can deliver. To compare the total output of light by two brands one has to see the Lumens offered by each product. Reputed and reliable manufacturers will ensure that their products show the highest Lumens reading throughout the running time of the battery.
Luminous Flux and Lux
1 Lux = 1 Lumen / m2. Hence Lux is concerned with the area also. From the Lux reading we come to know the intensity of the light delivered by the product in a specific area. In other words Lux indicates the intensity of the light on a surface. Total amount of visible light present on a specific area is measured as Lux. Lux enables us to perfectly measure the brightness of a beam. The amount of light that can be provided by a flashlight or a headlamp depends on the amount of Lumen it delivers. Lumen is also known as luminous flux which is unit to measure the quantity of light. Both Lumen and Lux are used for measurement of light but, the difference is that while Lumen deals with the beam pattern Lux is the unit to measure the intensity of light by taking into account a specific area.
The amount of light delivered on a specific area is measured as Lux. In other words Lux indicates the brightness of the beam whereas Lumen shows the beam pattern. When the light is spread over a small area it will appear brighter. The light is weak when it is spread over a large area. In order to control spreading of light so as to create a beam of a specific pattern we use reflectors, optics and mirrors. Ambient lighting as well as decorative lighting will be of low Lux value since the light will have comparatively larger footprint, though configured to travel shorter distance. Lux Meter is used to measure Lux at a specific area.
Lux indicates brightness of a beam
The difference between Lumen and Lux is that Lux is considered with the area over which the Lumen is found. Lux indicates the illuminance. When 1000 Lumens of light is spread over an area of one square meter, the illuminance of that one square meter area is 1000 Lux. When 500 Lumens can illuminate 1 square meter area with 500 Lux, 500 Lumens can illuminate 10 square meter area with 50 Lux only. Since Lumens indicate the quantity of light delivered by a source, when the Lumens are more the light will be brighter. Lux is considered with the area of the surface on which the light is delivered. When Lux is higher the indication is that over a specific area the light is brighter. In other words Lux indicates intensity of the light over a specific area. Lux precisely shows the brightness of the beam. When a beam is spread over a larger area, it gets diffused and will appear as dim light. The amount of light delivered over a specific area is represented in Lux. Since one Lux is equal to one Lumen per square meter, Lux shows the amount of visible light as well as the intensity of the light over the specific area. While purchasing lighting products one should check Lumens and Lux also to ensure that the product can meet the requirements of the buyer.
Luminous efficacy of a lamp
We should understand that Lumens reading is not an indication of the quantity of power that the lamp consumes. The Lumen output by the lamp per watt of power consumed by the lamp will show its luminous efficacy. The output by the lamp in terms ofLumens shows the Luminous flux. Luminous flux is the total quantity of light provided by the lamp. The efficiency of alamp is determined from the number of Lumens output by the lamp per Watt consumed. The highly efficient sodium lamps deliver above 100 Lumens per Watt consumed whereas the incandescent lamps provide less than 20 Lumens per Watt consumed. Those who want to buy lamps must be aware of the fact that the illuminance of a lamp will not be included in its specifications because illuminance is the output by the lamp at a particular place where it is used at the particular point of time to illuminate that area. When 4000 Lumens delivered by a lamp illuminate a surface with an area of 20 square meters, the lamp gives an illuminance of 200 Lux and when the same lamp of 4000 Lumens is used to illuminate a surface of 10 square meters, the illuminance will be 400 Lux.
What is Brightness?
Those who purchase headlight lamps are very particular about the brightness of the headlight bulb. While buying headlight lamps a clear understanding of the correct headlight brightness is all the more important. Both Lumen and Lux are to be taken into account while determining the brightness of a lamp. Luminous flux which is measured in Lumens shows the total quantity of visible light provided by the lamp and the Illuminance which is measured in Lux shows the intensity of the light delivered by the lamp up to a particular distance.
Since brightness is a subjective attribute, one cannot measure it objectively. Since brightness indicates how we observe an object it is a subjective attribute and hence brightness need not be proportional to luminance. Since brightness is a relative term, it is the amount of light output by a source of light relative to another source to which we compare it. When the light output by Source B is more than the output by Source A we say that Source B is brighter than Source A. Similarly, if Source C makes more output of light when compared to Source B, we say that Source B is less bright and Source C is brighter.