Not knowing how thick the glass is, what the tint is, whether it's fabricated into an insulating glass unit, and whether there are any reflective coatings involved (I'm a glazing contractor), I can't tell you exactly what % of the visible spectrum is getting through to your plants, but it's almost certainly less than half, and perhaps as little as 25%

Supplemental lighting can be helpful, but remember light intensity diminishes by the square of the distance from the source. Example - the amount of light striking a given surface area oriented to maximize exposure would be 1/100 the amount at 1" compared to the measurement at 10". At 2 feet (24"), it would be 1/576 that at 1".

Al