Do you know the light requirements for your plants? I thought orchids were quite low light?

Anyway, an HPS in your greenhouse will produce the equivalent light intensity to bright shade at the edges of your greenhouse, a little brighter in the middle. This is also roughly equivalent to direct light on an overcast (not heavy cloud) day in summer.

HPS is probably the best choice for you. It produces the most light of any widely-available lamp and is good for supplementing natural light. The only reason to choose anything else is that HPS light on its own is not suitable for some types of growing.

Paint the bricks white for brightest light. Leave them bare for maximum heating.

Thank you shrubs n bulbs.
Orchids' light is meausred in foot candles.
Different species require different amount of light.
The lowest require is for about 1,000 fc; the highest is about 4,000 fc. This will be quiet tricky to arrange those plants because they are on 4 shelves, one on top of the other.
Thanks for the info.

The HPS alone will produce roughly 1,000-2,000 foot-candles depending on where you position the plants under the lamp.