Measuring Artificial Light without a Quantum PAR Meter

Some of the feedback I heard from my previous articles was: "OK, you gave me some great info about PPFD, but I dont have a PAR meter! How can I put those numbers you gave me to use?" Lucky for you, here are a few ways to use PAR data as a home grower. 

1. Buy a lamp that advertises its brightness in PPFD. 

There is an amazing variety of specialty grow-light bulbs on the market today. With a little digging, you can find one that will advertise its brightness in PPFD. The most common way lower-end companies advertise their bulb's brightness is in a unit called lumens, or lux. Lumen is a unit of brightness which favors wavelengths that human eyes see best, and ignores a great deal of photosynthetic light. Buying a light with a high lumen output does not necessarily mean that your plants will receive much light they can photosynthesize from. This is explained in depth in my article Why We Use PAR and PPFD for Growing Orchids and Houseplants. 

For example, I took my quantum PAR meter (we have the Apogee MQ-500) into a bathroom without any windows. The only light source in the bathroom was a three bulb fixture with typical fluorescent 40-watt bulbs. I put my PAR meter 8 inches or so below the bulb and got incredibly low PPFD readings. The readings varied from 4-6 PPFD, well below any light level I would recommend for growing even the shadiest plants. These bulbs appeared bright to my eyes; they illuminated the entire room. And yet there was hardly any photosynthetic light emitted from them. Therefore, as an artificial light grower, you should avoid buying bulbs that only advertise their brightness in lux or lumens. The brightness of the bulbs can be deceptive to our human eyes! Conversely, one comment I often hear about our grow lights at the HDO growhouse is how dim they seem. And yet, I'm growing very high-light loving Brassavolas and bifoliate Cattleyas successfully and blooming them beautifully. Don't trust your eyes to measure how much PAR your plants are getting. 

For research purposes, I looked pretty carefully at these Barrina LED lights. I was interested in this particular product because they have a nice diagram mapping out the brightness of their bulbs in PPFD. I looked at the version with the reflectors, just like in the image below. 

Figure 1. Barrina Marketing Material PPFD Values (Source: Barrina Marketing Materials)

I mapped out the PPFD values using our PAR meter just like they have in their marketing materials. I elevated the fixture above a grid on the floor and took measurements just as had been outlined in the Barrina marketing diagram.

Here's our results:

 Table 1. Barrina LED PPFD values as measured by Kelly 

As you can see, the values I measured were 18-32% lower than the advertised values. I did check the calibration on my PAR meter to ensure the meter was working properly, and I emailed the company asking for an explanation. Barrina simply stated that their experiment was done in laboratory conditions and offered no further explanation for the discrepancy. The “laboratory conditions” they speak of is probably the fixture being sealed in an integrating sphere. But you’re not growing in an integrating sphere, you’re growing in your living room, such as how I had my experiment set up. Because of this, I recommend buying slightly more light than you actually anticipate using. You can easily reduce light by elevating the bulbs, but it's a little more difficult to add more light later on. Some higher-end LEDs also offer dimming feature which can be useful.

Even though the advertised brightness was slightly higher than what we measured, this is still incredibly valuable information. As I mentioned above, brightness of bulbs can be deceptive, appearing bright to our eyes, but providing very little photosynthetic light. Having some information about the expected PPFD output of your bulbs is better than the incredibly useless lumen output that most fixtures advertise. 

2. Use my Poor-Man's PAR Meter 

For info on why you can't use lux to measure artificial light directly, please read our article on PAR and PPFD to measure artificial light. You can however, convert lux into PPFD, based on the spectrum of the bulb you are using. You will need a lux meter; your smartphone can work for this. 

To test this method, I used an app called Photone, and an iPhone 11 Pro Max. The Photone meter app requires a diffuser to work properly. If you don't have a professional diffuser, the app recommends cutting a strip of paper 1/2" x the width required to wrap around your phone. (I used Amazon basics standard office paper.) Tape the strip of paper around your phone, over the camera, and voila you have a diffuser. This sounds totally sketchy, but to my surprise it actually worked.  

Make sure you are measuring in lux, not foot candles. Using the Photone app with the diffusing paper, take a lux measurement and multiply it by the conversion factor in the table below. I included several lights in the table, mostly for illustration purpose, to show the how the conversion factor changes based on the spectrum emitted by each bulb. 

 Table 2.  Lux to PPFD Conversion Factor

The conversion factor in Table 2 is specific to the spectrum that the light produces. In order to apply this to a different light, you can calculate your own conversion factor from the equation below: 

The power unit of the f(λ) curve doesn't affect this calculation, since f occurs in both the numerator and the denominator. For example, you can use a normalized f(λ) curve (where the maximum is set to 1.0), which is what the light manufacturers typically advertise. 

We tested the conversion on the Barrina LED bulbs and our PhotoBio bulbs. As you might expect, it's not perfect. The Barrina result was accurate to within about 15%, while the PhotoBio result was accurate to within about 5%. Since the accuracy of this equation depends heavily on the spectral data, and an iPhone with a piece of paper taped over it is hardly a precise instrument, these readings will never be perfect.

Even this imperfect information can help you avoid a situation where you have considerably insufficient light. Orchids and houseplants often don't reveal the effects of insufficient light for many months.

3. Use Photone’s PAR Meter

Photone does offer a paid add-on PAR meter. It requires the diffuser, and still isn’t a perfect instrument. Again, this can help you avoid a situation where you have too little light and can get you in the ballpark of where you want to be. Photone is likely using a similar conversion factor equation to mine above (Figure 3). As the conversion factor changes drastically depending on the spectral data, you will not get a perfect result, and it will be less accurate than calculating your own conversion factor.

Here are some measurements taken with the Photone app and my Apogee Quantum PAR meter, to compare the accuracy.

4. Use a lux meter or phone PAR meter to produce a relative intensity map of your bulb

Don't have a PAR meter and you're wildly confused by our conversion factor equation? That's ok. You can still use your smartphone to give you a good deal of information about the relative brightness of your bulbs. With pretty much any light fixture, the light emitted from it is going to have bright spots and weak spots. You can plot these out using a lux meter and place your bright-growing plants in the places with the highest readings and the dimmer growing plants in the places with the lowest readings. 

Since you can only evaluate the relative intensity, this data is only useful if you are comfortable with your bulbs and know what you are able to grow under the brightest parts of the bulb. This data wont help you avoid a situation where you have drastically too little light. But, if you have grown (and flowered, if you're growing orchids) a light-loving plant under that bulb, you can better understand the relative bright and dim spots produced by that fixture. 

For information about what we are growing at what PPFD value, read this article: Target PPFD for Orchids and Tropical Houseplants. 

Feel free to send us an email at kelly@highdesertorchids.com with any questions.