What is the difference between 655/650nm and 638nm red?

Professional diode laser systems are usually equipped with 638nm red diodes. These have a very good visible red color and can be proper collimated to good beam shapes. The handling of these kind of diodes is quite tricky, especially if there is the need of combining more than two diodes. We use this diode type in our professional laser systems. Most Chinese manufacturers are not able to handle this kind of diodes.

The 655/650nm red is easier in handling and cheaper, although the beam profile is worse. This kind of diodes is normally not suitable for optically combining more than two single diodes, that's why the red power is somehow limited. We use this diode type in our budget systems to have a very good price-performance-ratio. The red color is somewhat deeper.

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Below you can see the the visibility curve of the different color spectrums. Lime color is at best visibility point, whereas red and blue ar worse visible.

The red and the blue color react completely different if mixed with green (which is done in RGB color mixture): Whereas blue (445nm( can be smoothly enriched with green and gains much visibility with only having small portions of green added (and smoothly turns to different cyan tones), the red color very soon turns into orange and yellow if mixed with green.