Just do a simple search on ‘height of pendant light over table’ and you will find tens of thousands of result. After visiting most of them, you will realize you need to hang your lights between 2.5ft to 3ft from the table.
The question is BUT WHY? How does one universal solution fits all? What if you are hanging them in kids room, what if you are hanging them in ladies hostel. What are the factors that affect - will seating height affect, does the lamp size affect, so on and so forth.
Welcome to the ergonomic approach of hanging your lights. In this article, you will exactly know why and what factors influence the hanging height of your lamps, and when you know them, you will be able to call on the importance of the various factors to be able to adjust your hanging height in difficult scenarios like, low ceiling, tall members in family, low/high seating arrangements, only for seats and no table etc.
Assumption
The basic assumption on which this is based is that the lamp should clear the view or peripheral line of sight while seating as someone will not want something bright in view that draws attention constantly while seating which practically, means the inner side of the lowest part of the lamp which is away from the user should clear the line of sight as in the image shown in the right.
Inferences
From this figure: the upward view angle for humans is approx. 25°, so keeping that in mind, there are few inferences that we can draw:
The height of hanging depends on
1. The width of lamp opening
2. The width of table
3. The sitting height of person
Further, you can infer the following based on the above figure
Wider the lamp = higher the lamp
Wider the table = higher the lamp
Taller the person = higher the lamp
Calculating the height
Finding the eye height (H1)
To get to the height of lamp, lets start calculating. 1st step is to calculate the eye height.
For a public space, you can use the image below which takes care of seating height and the general population height, however, for a home, you can just seat the tallest person at the designated space and measure the eye height directly from the ground.
Affect of table width (D) Here, we are interested in the horizontal distance of the center of lamp to the eye point assuming you plan to hang the lamp in the center of the table,
The distance of eye point will be half the table width + 3" (assuming you sit a bit away from the edge of the table. You can change this 3" to any value, based on your habits of sitting at table).
Once we know 'D' we should be able to calculate the height of lamp (without considering the width of the lamp).
Calculating the height above eye level (H2)
To find the height, we will need to use a bit of trigonometry as we know the angle of the view (that should be cleared) so the height will be D X Tan 25° = D X 0.47
The minimum height of hanging
if you ware to omit the width of lamp or are really short of vertical space you can consider the below formula
EYE HIGHT + 0.25 x Table width
as the minimum height that you can hang your lamp
I have arrived at above as following
Eye height - as measured
H2 = w/2 x 0.5
In above, I have taken 0.5 instead of 0.47 and taken table width W instead of D (as higher value of D will be compensated by higher value of Tan 25 )
Considering the lamp opening dia. (H3)
Now, if we were to consider the lamp diameter too. This becomes a considerable factor when using larger lamps.
To calculate H3, we can use the same formula as above
where H3 = L/2 x 0.47 (where L is the diameter of opening)
Just to make life simple, we will consider
H3 ~ L/2 x 0.5
so, the final height of lowest portion of the lamp can be arrived at
H1 + H2 + H3
where
H1 = eye height of the tallest person sitting at the designated space
H2 = 0.25 times the width of table
H3 = 0.25 times the lamp opening diameter
Hence, your final formula to calculate the height lowest part lamp from the ground will be:
Height of lowest pt. of lamp = sitting eye height of tallest person + 1/4th of table width + 1/4th of lamp opening diameter
considering you are hanging the lamp in the middle of table. Do make sure that you use same units for all three heights.