Why LED Tube having same lumens but different lux?

The lumen (symbol: lm) is the SI derived unit of luminous flux, a measure of the total quantity of visible light emitted by a source. Lumens are related to lux in that one lux is one lumen per square meter.

The difference between the units lumen and lux is that the lux takes into account the area over which the luminous flux is spread. A flux of 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux.

The same 1000 lumens, spread out over ten square metres, produces a dimmer illuminance of only 100 lux. Mathematically, 1 lux = 1 lm/m2.

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Fig. 1 Lumens vs lux comparison pictorially. Lux changes with beam angle and height. Lumens is measured in a sphere. Lux can be measured by a lux meter.

 

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Fig. 2: Lux relationship with beam angle, power at the same distance

 

Fig. 3 Same 50W MR16 with different beam angles resulting in different lux levels. Lux will be lower further away from the center at the same distance.

 

Fig. 2 depicts the relationship between lumens, lux and beam angle at a fixed distance. A lamp may have the same total lumens; however, depending on the beam angle which could be shaped by collimator or reflector; may result in different lux intensity.

Fig. 3 shows the center lux of a 50W MR16 with different beam pattern. The 10 degree beam will be brightest in the center; however, the lux drops very fast away from the center. Thus, it is wrong to conclude that 10 degree beam MR16 is brighter than the 60 degree beam MR16 and therefore it is a better light.

The total lumens is the same for all the four lights. The 60 degree beam has low center lux because it has more light spread over a larger area. The 10 degree beam is good to provide spot lighting. The 60 degree beam may be good for different lighting ambiance.

One simple test  is to take the lux reading at various locations away from the center of the lamp.

In a traditional omni-directional lighting like HID or fluorescent tube, secondary reflector is added to do the beam forming. In LED, the LED component has a 120 degree beam pattern.

Fig. 4: Cross section view of the LED tube. Depending on the depth of the LED components, it may have different effects of the tube beam pattern.

 

For a clear cover tube, the beam pattern is 120 degree. For diffused tube, the effect may be different due to the surface lighting.

Fig. 5: MY LUMENS tube uses specialty material to transform the light for better linear lighting effect.

 

The wider side beam of MY LUMENS tube enable secondary reflection on the luminaire aluminum reflector; thus, less shadow near the ceiling. The whole room lighting effect will have lesser hot spots. In the replacement market, LED tube is being installed into a reflector designed for 360 degree beam lamp. LED is a directional lighting thus enabling different lighting effects.

 

Fig. 6 Fluorescent Tube and LED Tube Coefficient of Utilization in the same specular reflector system

 

Fig. 5 shows that collimator or reflector is part of the LED lamp; thus, when comparing LED lamp; it is important to consider the beam angle; otherwise, the lux comparison may lead to the wrong conclusion. A narrow beam lamp will have higher lux at the center. When implemented, there will be dark spots between lamps. The effect will be worst near the ceiling.

 

Fig. 7: Luminaire beam angle effects. 3rd picture shows serious shadows.

 

 

MY LUMENS’ Lunar series adopted an indirect lighting approach to lower the glare and spread the brightness.

Fig. 8. Lunar is a complete troffer to improve the Coefficient of Utilization. This reduces light loss absorbed by the reflector. In fact, Lunar has 98% reflectance.

 

Fig. 9: Two different LED light systems on the same position, height along the same corridor. Notice the shadow effects.

 

The legacy troffer is designed for 360 degree tube. The T8 LED tube is good for replacement market. For new building, a more efficient integrated luminaire similar to Lunar series may be a better choice.

 

Conclusion: For bench marking, the lm/W is the appropriate approach to compare lightings. Lux/W is the wrong way to compare. Beam angle plays important role in beam shaping for different lighting applications.