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The Physics of Precision: Wavelength and the Diffraction Limit<\/h2>\n\n\n\n
To understand why a blue LED for encoders<\/strong> is so critical, we have to talk about how an optical encoder actually reads position.<\/p>\n\n\n\n Whether you are designing a transmissive or reflective system, you are essentially projecting light through or off a fine-pitched grating (the code disc) onto a photodetector array. The photodetector measures the resulting light pattern and translates it into sinusoidal electrical signals, which are then interpolated to calculate position.<\/p>\n\n\n\n The problem? Light does not like to travel in perfectly straight lines when it passes through tiny gaps. It bends. This bending is known as diffraction.<\/p>\n\n\n\n The angular spread of this diffracted light can be estimated by a simple physical relation:<\/p>\n\n\n\n Where:<\/p>\n\n\n\n If you look closely at this formula, you will notice that the diffraction angle When you use an infrared light source with a wavelength of 850nm, the light spreads out significantly as it passes through the narrow slits of a high-resolution grating. This causes the light and dark patterns on the sensor to blur together. The resulting electrical signal loses its contrast, noise creeps in, and your interpolation accuracy goes out the window.<\/p>\n\n\n\n Now, look what happens when we swap that out for a 465nm blue LED for encoders<\/strong>:<\/p>\n\n\n\n By shrinking the wavelength from 850nm to 465nm, you cut the diffraction angle almost in half! The light beam stays much tighter and sharper as it travels from the code disc to the sensor. This means your sensor recieves a highly defined, high-contrast optical pattern.<\/p>\n\n\n\n Here is a quick comparison of how wavelength impacts different light sources used in encoders:<\/p>\n\n\n\ntheta = lambda \/ w<\/code><\/p>\n\n\n\n\n
theta<\/code> is the diffraction angle (the spread of the light beam)<\/li>\n\n\n\nlambda<\/code> is the wavelength of the light source<\/li>\n\n\n\nw<\/code> is the slit width of the grating on your encoder disk<\/li>\n<\/ul>\n\n\n\ntheta<\/code> is directly proportional to the wavelength lambda<\/code>.<\/p>\n\n\n\n\n
lambda<\/code> = 850nm<\/li>\n\n\n\nlambda<\/code> = 465nm<\/li>\n\n\n\nParameter<\/th> Traditional Infrared (IR)<\/th> Standard Red LED<\/th> 465nm Blue LED<\/th><\/tr><\/thead> Typical Wavelength<\/strong><\/td> 850 nm<\/td> 640 nm<\/td> 465 nm<\/td><\/tr> Relative Diffraction Spread<\/strong><\/td> 100% (Baseline)<\/td> ~75%<\/td> ~55% (45% Reduction)<\/strong><\/td><\/tr> Signal Contrast (Modulation)<\/strong><\/td> Low on fine pitches<\/td> Moderate<\/td> Very High<\/strong><\/td><\/tr> Max Physical Resolution<\/strong><\/td> Limited (requires large discs)<\/td> Medium<\/td> Excellent (allows ultra-fine pitches)<\/strong><\/td><\/tr> Common Application<\/strong><\/td> Legacy industrial encoders<\/td> Low-cost consumer encoders<\/td> Ultra-precision, compact encoders<\/strong><\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
![\"465nm](\"https:\/\/photo-detector.com\/wp-content\/uploads\/2026\/02\/E465-4-201L4-0-600x600.webp\")