I ran across them a few times before, but now I needed a bright light with a light quality close to sunlight.
Some of the newest LEDs can deliver that, but they are quite expensive (40€ for the LED itself. Driver and heatsink not included).
Then I found a used 70W metal halide lamp with 44° reflector, housing and driver on ebay for 20€ and bought it. It is fantastic! (It hangs 150cm above a desk).
The light intensity 60cm from the lamp is approximately equal to that of the rare winter sunlight in the north. So at the desk it is about 1/4.
The color is a bit on the pink side compared to the sun (which is quite blueish at this time of year) and it takes a while to power up and stabilize in color.
If high quality LEDs are outside the budget (or cooling them is an issue – high power LEDs need a cooling system!), metal halide lamps are definitly a very good alternative (and about as efficient).
As I recall, metal halide bulbs were a key component of pre-LED indoor grow systems, utilised during flowering, after High-pressure sodium bulbs had been used for vegetative growth.
Both high-pressure sodium and metal halide bulbs put off an astonishing amount of heat, as in, if I had my 450W going, I didn't need supplemental heating except when it was truly frigid.
I don't have a chart with comparative energy use between the two aforementioned technologies and modern LED growbulbs, but I will bet that the ancient tech is way more expensive to operate than the LEDs, and with a shorter lifespan, to boot.
There used to be so much draw off the old systems and so much waste heat that the police used to look for both spikes in power usage and, in the winter, the houses in the neighbourhood that had the warmest heat signature, often first signalled by the lack of snow on the roof of a house whose neighbours' were all buried.
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein
What does a metric clock look like? I bet it is nothing like this tiny ad: