I meant 5000 lumen!  not 5000 K.

Thanks for your question. It is always fun to start growing seeds at this time of the year (early March 2023). Helps with our cabin fever!

You have asked an important question regarding lumens. Lumen requirements depend upon the types of plants you are growing so I will only be able to provide you with a rough guess. For growing plants indoor, the optimal light is 7,000 to 7,500 lumens per square foot. However, for plants requiring just minimum lighting, roughly 2,000 lumens per square foot should be fine. Now in your case, you are using 5,000 lumen lights for an area of 2 feet by 4 feet. That is 8 square feet requiring a minimum of 16,000 lumens. You may be pushing things with respect to light exposure as it now stands. Two suggestions. Use two or three lights next to each other. (tandem). You may have to apply some of your electrical skills to achieve this. I assume that you are able to adjust the height of your lights. Lower them down to just three inches or so above the plants. This will reduce the lumen requirement. Of course then you will need to do due diligence and as your seedlings grow upwards, raise the lights appropriately.

Fun to do this. Please let me know if you have further questions.

Thanks for your recent question. It is highly appropriate. 

There are various technical definitions of a lumen. When I taught plant physiology at the UM, which included discussions of photosynthesis, I used a rather crude means by which to define a lumen. Surprisingly it is quite accurate. One lumen is approximately equal to the amount of light put out by a candle when viewed a foot away. In other words, a bulb rated for 5,000 lumens is a measurement of light approximately a foot away from the bulb. As you raise your grow lights up from the plants, the lumen intensity decreases since the light is being dispersed over a wider area. Conversely, as you decrease the distance between the light and the plants the lumen intensity increases. Without getting too much into the weeds but I do greatly appreciate your interest, light intensity adheres to what is called the inverse square law. Simply stated, this means that light intensity is inversely related to the square of the distance from the light to the plant. You were quite correct in your response when you mentioned an exponential relationship. Examples: A 5,000 lumen bulb will deliver 5,000 lumens of light to plants a foot away from the bulb. If the distance is increased to two feet away, the amount of light is diminished by (1/2) squared or 1/4. If the distance is decreased to half a foot, the amount of light on the plants is increased by (1/0.5) squared or four times as much as compared to a distance of a foot. This would then mean that your 5,000 lumen bulb is now delivering 20,000 lumens of light.

Please forgive me if this is far more technical than necessary. Certainly feel free to get back to me with any additional questions. Good luck. Enjoy your plants.

Thanks for the response.  I really don't think there is need for any concern.  Those lower leaves should still have their light needs met.  The top leaves should also be fine.  They can never get too much light.

Thanks again for using our forum.