Opal shopping guide

In article of opal buying guide, I have read this , if opal is Very Bright in brightness of fire, it means, shows good color under low light and sharp crisp color under indirect sunlight. What does it mean , Indirect sunlight? is is the room light equivalent or what is the low light term in this article?
Please help

“Indirect sunlight” means the light in a room illuminated by natural light through a window, not directly exposed to sunlight. Donald Clark, the author of the opal buying guide, recommends using the light from a north-facing window or a shaded window facing any direction for color evaluations in his article on gem color. Although filtered sunlight is the ideal choice, he does suggest as an acceptable alternative incandescent light, either standard or daylight equivalent.

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Thanking you for being so responsive , Merry Xmas.

Just to clarify, although you can observe opal brightness of fire with either indirect sunlight or incandescent light, if you are viewing or testing for color change gems, sunlight and incandescent light may produce different results.

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