Because achromats and planachromats are best corrected spherically for green light, and since the use of monochromatic light eliminates chromatic aberration, the performance of achromats is markedly improved with the use of a green filter. Also, phase contrast objectives are computed to give best phase images in green light.
Human eyes see the colour green the best. And, since monochromatic light eliminates chromatic aberration, a green filter markedly improves the performance of HC. In addition, phase contrast objectives give their best images with green light.

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