Power Versus Intensity when Choosing LED and Light Delivery

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What is the difference between power and intensity? This concept comes up often when speaking with customers and is crucial for choosing the best LED and light delivery option.

Here is the difference: 
Power, or total power, describes how much energy is emitted from the light source. 
Intensity describes how much power there is per area. 

Not every high power light source can be used to deliver intense light to a small target. Due to the laws of physics, light from a large emitter cannot be concentrated on to a small spot or squeezed into a small fiberoptic. The rule is that light at the target can never be more intense than it was at the original source. 

Prizmatix LEDs come in different models that have different size emitters. (Learn more  here

Smaller LEDs tend to have less power than larger LEDs, but they have higher power per area. Larger LEDs have less power per area, but they have higher total power. And smaller LEDs are less divergent than larger LEDs so they can be focused down to smaller spots. 

To illustrate click here to open our Spectraviewer to see 3 blue collimated LEDs.

Product P/N Total Power [mW] Emitting Area Intensity mW/sq mm
UHP-T-450-SR 2100mW 2 sq mm 1050 mW/mm
UHP-T-450-EP 2400mW 4 sq mm 600 mW/mm
UHP-T-460-DI 4400mW 9 sq mm 488 mW/mm

As you can see the higher power LEDs have double the power but less than half the intensity. So, for applications that need bright light, the lowest power LED will be the best choice

When comparing LED that are coupled to fiber optics or light guides, the same rules apply A 5mm LLG will typically give you higher power but less intensity than a 3mm LLG. And a smaller fiber optic may be even better. 

A 5mm Liquid Light Guide will usually deliver double the power of a 3mm LLG. But the emitting area of a 5mm LLG is more than double that of a 3mm LLG. So the intensity is higher with the 3mm LLG. 

Here is one example from our  UHP-F page.

Light Guide Size Total Power Area at exit of LLG Intensity mW/sq mm
UHP-F-405 5mm 6000mW 19.625 sq mm 305.7 mW/mm
UHP-F-405 3mm 2800mW 7.065 sq mm 396.3 mW/mm

When is Total Power important? 

Total Power is important when the target is relatively large. When the target is large and the requirement calls for a certain number of mW per cm, high total power is what you need. For example, a microwell plate has about 108 sq cm. If your application calls for 30mW per cm, you will need 30x108 = 3240mW or 3.24 watts. Now if the light is coming from a light guide, you will need a circle big enough to cover a microwell plate. That circle would need to have a diameter of about 15.5cm. The area would a spot of 189 sq cm. For 30mW/cm sq you would need a circle with a total power of 5650mW.

When is intensity important? 

Intensity is important when you want to get a lot of light in a small space. Two prime examples are when coupling to fiber optics or delivering light through a microscope objective. For applications such as in-vivo optogenetics or fluorescence imaging, intense light is much more important than total power. So don’t be fooled by LED with 4 or 5 watts of power. If the LEDs are large, you won’t be able to utilize that power. A smaller LED with 1-2 watts is what you need to use.

Choosing an LED: 

So which LED should you choose? For high power the UHP-T-DI, MP and LA models are the best choice. The emitter size of the LEDs are 9-12 sq mm. For light guide LEDs, use a 5mm LLG. For highest intensity, the UHP-T-SR is usually the best choice. And for applications that direct illumination is not possible, a 3mm LLG or a fiber optic. The size of the fiber optic depends on how it will be used and can vary from 100um to 1500um. The UHP-T-EP is in the middle and a good choice when you need a balance of power vs intensity. Our sales consultants are happy to help you choose the best LED for your application. Contact us to discuss your light source requirements.

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