Table 1. Determination of the absolute η _uc using data from the calibrated epi-luminescence imaging of single and small clusters of UCNPs.

	Pem [W] (25%)	NYb (10%)	σabs [cm2] b)	Iex [W/cm2](10%)	ηuc [%]
Cluster 1a)	2.6×10−14	5.5×105	1.0×10−20	2.6×102	2.0±0.5
Cluster 2a)	2.2×10−14	6.1×105	1.0×10−20	2.6×102	1.3±0.3
Singlea)	4.9×10−15	2.3×105	1.0×10−20	2.6×102	1.2±0.3

^a)As identified in Figures 3B and 3C. ^b)The Yb³⁺ absorption cross-section, as in Refs 15, 26. Unites are given in square brackets, percentage standard deviations are given in brackets, except for η_uc.