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Our eyes have both cone and rod cells. Rod cells measure the intensity of light whereas cone cells identify the colour of the image formed in the eyes. So cone cells must also be able to identify white, black and grey and also their shades.

So it wouldn't have been that much trouble to just have cone cells cells as identifying black and white would have been the same as measuring the intensity of light. Wouldn't it?

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    $\begingroup$ Duplicate of biology.stackexchange.com/q/42216/3340 $\endgroup$ – WYSIWYG Jan 12 '16 at 9:45
  • $\begingroup$ @WYSIWYG this question is different $\endgroup$ – HDatta Jan 12 '16 at 9:52
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    $\begingroup$ The question might sound different but the answer there completely answers your question. There is no point in repeating the same answer, isn't it. If there is something that is missed out in that answer that your question pertains to, then you can add some details to clarify. $\endgroup$ – WYSIWYG Jan 12 '16 at 9:55
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    $\begingroup$ Please see Christiaan's answer to Jen's question. $\endgroup$ – WYSIWYG Jan 12 '16 at 9:57
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    $\begingroup$ It is clearly mentioned in Christiaan's answer that rods are required for vision in low-light conditions. $\endgroup$ – WYSIWYG Jan 12 '16 at 10:08
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First off - both the rod and cone system measure light intensity; both sensory cells increase their responses when excited by more photons (Purves et al., 2001). Hence, your introductory sentence that rod detect luminescence and cones color is incorrect. They are both sensitive to luminescence.

You are correct that rods detect gray scales, while cones perform color coding. Both systems can detect gray shades; while rods are limited to that, cones are not. Rods are very sensitive to light and in a dark-adapted state are capable of detecting a single photon. Hence, they operate under conditions of low-lighting at night and as such they mediate night (scotopic) vision. Cones are photon-hungry and need more light to operate (photopic conditions).

  • Why have cones? Because color vision helps to detect and differentiate stuff.
  • Why have rods? Because night vision is helpful.
  • Why have both? To be successfully equipped to thrive in both light and dark environments.
  • Do we need them both? No - even nocturnal hunters that don't have rods have been shown to be able to hunt at night. Not only that, these geckos have been shown to be able to detect color in starlight (Röll, 2000). Under these lighting conditions, human retinae rely on the rod system and are unable to detect color.

References
- Purves et al., Neuroscience, 2nd ed. Sunderland (MA): Sinauer Associates
- Röll, J Neurocytol (2000), 29(7): 471-84

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  • $\begingroup$ But in night couldn't we have managed off with just using grayscale vision with the help of cone cells to view images. During night the cones don't work? $\endgroup$ – HDatta Jan 12 '16 at 11:03
  • $\begingroup$ our cones don't work at night. $\endgroup$ – AliceD Jan 12 '16 at 11:17
  • $\begingroup$ So, as cones don't work at night, we need rods to help us with grayscale images. $\endgroup$ – HDatta Jan 12 '16 at 11:31
  • $\begingroup$ @Reptile1234 It's more that you need rods to help you see at night. Not having functional rods results in nightblindness. As you can imagine, not being able to see at night will affect one's ability to survive. $\endgroup$ – C_Z_ Jan 12 '16 at 16:36

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