Table 1. Series of calibrated ND filter sets and respective OD values.

Set Number	ND filter combination	filter OD	Pin (pW)	cps (photon/s)
1	#2 + 4 + 5 + 7	4.099	165.6	4.17 × 108
2	#7 + 8	4.607	51.41	1.29 × 108
3	#6 + 7	5.009	20.37	5.12 × 107
4	#1 + 5 + 6 + 7	5.510	6.43	1.62 × 107
5	#1 + 5 + 6 + 8	6.089	1.69	4.25 × 106
6	#1 + 4 + 6 + 8	6.581	0.556	1.39 × 106
7	#1 + 3 + 6 + 8	7.030	0.194	4.90 × 105
8	#6 + 7 + 8	7.602	0.052	1.32 × 105
9	#5 + 6 + 7 + 8	7.976	0.022	5.68 × 104

For number of photon calculation, λ = 500 nm was assumed with input power of 2.08 μW.