Introduction of lamps and lanterns related parameters (I)

lamps and lanterns related parameters Luminous efficiency Luminous flux colour rendering index

1.Luminous efficiency

Luminous efficiency typically reflects how efficiently a light source transforms electrical energy into visible light. It’s usually expressed as lumens per watt (lm/W) and is an essential measure of a light’s energy conservation capabilities. A light source with a high luminous efficiency embodies strong conversion abilities, meaning it can provide the same brightness while using less energy or emit more illumination with the same power input.

Here are some typical luminous efficacies for common lamps: incandescent bulbs yield 8-14 lm/W; single-ended fluorescent lights offer 55-80 lm/W; self-ballasted fluorescent ones reach 50-70 lm/W; high-pressure sodium lamps deliver 80-140 lm/W; metal halide lamps produce 60-90 lm/W; and LED lamps achieve 50-200 lm/W. Notably, light sources for indoor lighting generally fall within the range of 50-120 lm/W, while those for outdoor street lighting often exceed 100-200lm/W. 


2.Luminous flux

Luminous flux is our eyes’ perception of radiant power – a product of specific wavelength’s radiant energy multiplied by its visibility. Since our eye perceives various wavelengths differently, the same radiant power doesn’t equate to identical luminous fluxes. Take green light at 555nm and red light at 650nm, both have equal radiant energy yet their luminous fluxes differ significantly.

The unit of luminous flux? The “lumen” (lm). This universal measurement is used globally, including in the SI and AS systems. The lumen is essentially a larger version of the electrical unit, the watt. However, since vision depends on light color, the lumen was chosen as the standard. Luminous flux also ties into the light source’s luminous efficiency. Our eyes are most responsive to yellow-green light at 555nm, where 1W equals 683 lumens. But for red light at 650nm, 1W yields just 73 lumens.

In essence, luminous flux measures how much light a light source emits over a given period, expressed in terms of visible radiation energy.


3.Colour rendering index(CRI)

Color rendering index (CRI) rates a light’s ability to replicate object color, often compared to sunlit color. Ranging from 0 – 100, a score over 80 generally suggests decent color replication. Lights with a high CRI make objects appear truer, therefore, preferred in settings like art galleries and boutiques.

Increasing the CRI helps us see the right shade of an object more clearly. Some places with high color demands, like museums, require a high CRI light source.

In essence, a higher CRI indicates superior color rendering and stronger color reproduction.

  • What could be the downsides of low color rendering?

(1) School classrooms must adhere to a minimum CRI of Ra80 for healthy lighting.

(2) Students may struggle to identify object color due to poor CRI, potentially leading to vision issues.

(3) Long exposure to poor CRI light can reduce cone cell sensitivity, causing eye strain and potential myopia.


  • Common CRI levels:

(1) Museums, art galleries typically need a CRI > Ra98 for vivid color reproduction.

(2) Indoor spaces require a CRI > Ra90 for accurate color judgement.

(3) Street lighting typically has a CRI < Ra80 for minimal color reproduction.


  • Why opt for a high CRI?/What advantages does a high CRI light source offer?

(1) High CRI means high color saturation and realism.

(2) It restores the true beauty of home decor and food color.

(3) Prevents vision issues like myopia, color weakness, and color blindness.

(4) High CRI light sources offer improved visual perception, aesthetics, and productivity.

High CRI light sources accurately reflect natural object colors, creating a visually appealing environment. Industries like retail, hospitality, and art galleries greatly benefit from them. They also boost productivity by improving color distinction, aiding in object identification. Industries like manufacturing, where color accuracy is vital, also stand to gain. Overall, high CRI light sources enhance the visual experience and productivity across various sectors.


  • Factors influencing a light source’s CRI:

A light source’s CRI (color rendering index) determines its ability to accurately reproduce object colors. Factors affecting the CRI include the light source type, light spectral distribution, viewing angle, and background color. Different light sources have varying CRIs. Incandescent lights have lower CRIs, while fluorescent lights have higher ones. Light spectral distribution also impacts the CRI. Narrow-band LED lights tend to have higher CRIs than broad-band LED lights. Viewing angle and background color also influence the CRI. The CRI can fluctuate based on the light source’s angle of view. Similarly, the CRI can be influenced by the illuminated object’s background color. Understanding these factors is key for accurate color reproduction in various applications.

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