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Solarmeter UVB & UVI Readings

- plus Ferguson zones -

Solarmeter 6.2 for UVB, Solarmeter 6.5 for UV index

Solarmeter 6.2 and Solarmeter 6.5

Note: The UV chart on the face of the Solarmeter 6.5 is NOT the reptile specific Ferguson zones chart, but a UV index scale (1-15) for human use. Only the Zoo Med version has the Ferguson zones chart attached to it.

Readings

All outdoor UV readings on this page were taken in the southern half of the U.S. using Solartech's Solarmeters. These tables were created over a period of several years.

UVB was measured with the Solarmeter 6.2pic digital UVB meter. It measures UVB in microwatts per square centimeter (µW/cm²). All UV index (UVI) readings were taken with the Solarmeter model 6.5.

On this page, tables 1-4 show UVB only readings, tables 5-6 show UVI only readings, and tables 7-8 show combined UVB and UVI readings.

Solarmeter 6.2 vs 6.5

The newer thinking is that Solarmeter 6.5 UV index (UVI) meter is actually more useful for measuring reptile UVB lights than the traditionally used Solarmeter 6.2 UV meter. Solarmeter 6.2 reads the whole UV spectrum, while Solarmeter 6.5 is sensitive to the specific part of the UVB spectrum that helps create vitamin D3. Both meters are manufactured by Solartech Inc.

I have had my Solarmeter 6.2 UV meter for many years, but only more recently did I buy the Solarmeter 6.5 UVI meter. If you want to buy only one meter, choose the 6.5 model.

The Solarmeter 6.5 has several benefits. I can be used to check if a reptile UVB bulb is safe to use and to figure out the correct hanging height. Unlike Solarmeter 6.2, the 6.5 model can also be used to compare the output between different types and brands of UV bulbs, as well as between UV bulbs and the sun.

a.) Solarmeter 6.2 for UVB

Reptile keepers use this UV meter to track the decrease of output from UVB bulbs over time. In other words, it alerts you to change the bulb when the UVB output has fallen off too much.

Zoo Med's Digital Ultraviolet Radiometer (ST-6) is the same meter as Solartech's Solarmeter 6.2, just with a different cover coloring and Zoo Med's label on it. In short, ZM ST-6 meter = Solarmeter 6.2.

b.) Solarmeter 6.5 for UVI

This UV index (UVI) meter is utilized by tortoise keepers to measure the UV index of reptile UVB bulbs.

Zoo Med's Digital UV Index Radiometer (ST-7) is the Zoo Med labeled version of Solartech's Solarmeter 6.5. In short, ZM ST-7 meter = Solarmeter 6.5.

The Zoo Med version has a handy, reptile specific UV index chart, called Ferguson zones (1-4), attached to it. It allows you to check if the UV output of your indoor UV bulb is appropriate for your species. According the included instruction booklet, most tortoises fall in the zone 3 with recommended UVI from 1.0 to 2.6. Scroll down for info.

Important! The UV index scale (1-15) printed on the face of the Solarmeter 6.5 is NOT the Ferguson UV zones chart used for reptiles. This scale is for use by humans to help avoid harmful exposure to UV radiation.

UVB reading w Solarmeter 6.2

UVB reading on a sunny spring morning at 8 am DST. Solarmeter 6.2.

Table 1: UVB sky readings, spring

For measurements in this table, the UVB meter was pointed straight up at the sky (sky readings), but not directly into the sun (solar readings). Direct solar readings would be higher for UVB. Scroll down to charts with both sky and solar readings.

Early May - sunny all day:

Time µW/cm² Notes
8:00 am 30  
9:00 am 97  
10:00 am 165  
11:00 am 234  
12:00 noon 289  
1:00 pm 291 highest UVB = 291 (DST time)
2:00 pm 283  
3:00 pm 233  
4:00 pm 154  
5:00 pm 70  
6:00 pm 21  

Table 2: UVB sky readings, mid summer

Again, the Solarmeter 6.2 was pointed straight up at the sky, but not directly into the sun.

Mid July - mostly sunny, but cloudy when noted:

Time µW/cm² Notes
7:00 am 9  
7:30 am 20  
8:00 am 35  
8:30 am 68  
9:00 am 94  
9:30 am 160  
10:00 am 191  
10:30 am 102 cloudy
11:00 am 127 cloudy
11:30 am 120 cloudy
11:45 am 246 sunny w. clouds
12:00 noon 202 sunny w. clouds
12:30 pm 346 sunny w. light clouds
12:45 pm 356  
1:00 pm 356  
1:15 pm 360 highest UVB = 360 (DST time)
1:30 pm 348  
2:00 pm 327  
2:30 pm 305  
3:00 pm 275  
3:30 pm 232  
4:00 pm 191  
4:30 pm 152  
5:00 pm 109  
5:30 pm 72  
6:00 pm 47  
6:30 pm 23  
7:00 pm 12  

Table 3: UVB sky / sun readings, early spring

For the following two tables, two separate UVB readings were taken. First, the UVB meter was pointed straight up at the sky (zenith). This sky reading value is listed first in the table. Then, the UVB meter was aimed right at the sun (direct solar reading). This direct reading is the second UVB value in the table. In other words, the format is sky / sun.

Direct UVB reading shows the maximum UVB a tortoise can receive if basking in the full sun. Usually, the direct reading is higher than the sky reading, but this may be reversed around sunrise and sunset. For example, see the UVB readings at 7:30 am and 8:00 am

Temperature and humidity were also recorded. Temperature and humidity probe was placed in a shady spot just a few inches above the ground.

Early Feb - sunny all day:

Time µW/cm² Temp °F Hum % Notes
7:00 am 0/0 47 86  
7:30 am 4/2 47 85  
8:00 am 14/8 50 86  
9:00 am 46/57 57 80  
10:00 am 75/108 64 58  
11:00 am 133/164 70 45  
12:00 noon 151/193 75 35 highest UVB = 193
1:00 pm 141/187 71 43  
2:00 pm 92/125 72 44  
3:00 pm 60/88 73 39  
4:00 pm 22/28 72 40  
5:00 pm 4/2 69 40  
5:30 pm 0/0 68 42  

Table 4: UVB sky / sun readings, fall

Readings format is sky / sun.

Late October - sunny all day:

Time µW/cm² Temp °F Hum % Notes
6:00 am 0/0 52 73 dark
7:00 am 0/0 51 72 lighter
7:30 am 1/1 52 71 sunrise 7:12
8:00 am 8/6 55 71  
8:30 am 22/24 60 61  
9:00 am 38/53 63 50  
10:00 am 84/117 66 43  
11:35 am 156/186 67 42  
12:00 noon 172/201 69 46 highest UVB = 201
1:20 pm 179/189 71 43  
2:00 pm 165/187 70 44  
3:35 pm 72/86 73 33  
4:00 pm 41/73 72 33  
4:50 pm 20/11 71 33  
5:10 pm 10/9 70 33  
5:35 pm 3/2 70 35 sun low

 

Solarmeter 6.5 for UVI

Solarmeter 6.5 for measuring the UV index (UVI).

Table 5: UV index (UVI), winter

These UV index (UVI) readings were taken outdoors with Solarmeter 6.5 UV index meter in the southern part of the U.S. during the shortest day of the year (Dec 21). The meter was pointed at the sky. Sunrise was at 6:59 am and sunset at 4:51 pm The weather was a cloudy most of the day.

Late December - mostly cloudy, but sunny when noted:

Time UV index Temp °F Hum % Notes
7:00 am 0.0 50 83 sunrise
7:15 am 0.0 50 83 sun up
7:30 am 0.0 51 84  
8:10 am 0.0 51 83  
9:00 am 0.2 54 84  
10:00 am 0.3 55 82  
11:00 am 0.5 57 81  
11:30 am 0.9 58 80 partial sun
12:00 noon 0.9 60 76 highest UVI = 0.9
2:15 pm 0.4 63 63 sunny
3:10 pm 0.1 63 61  
4:00 pm 0.0 62 69  

Table 6: UV index (UVI), winter

These UV index (UVI) readings were taken in the southern part of the U.S. day after the shortest day of the year (above). Since the previous day was so cloudy, I took new readings on this sunny day with a clear blue sky (Dec 22). Notice how the UV index readings somewhat are higher than on the cloudy day before.

Late December - sunny all day:

Time UV index Temp °F Hum % Notes
7:45 am 0.0 51 73  
8:10 am 0.1 53 72  
8:30 am 0.2 55 69  
9:00 am 0.3 55 69  
9:30 am 0.4 58 68  
10:00 am 0.7 66 44  
10:30 am 1.0 69 36  
11:00 am 1.3 70 34  
11:30 am 1.4 69 33  
12:00 noon 1.5 68 37 highest UVI = 1.5
12:30 pm 1.4 68 38  
1:10 pm 1.2 68 36  
2:05 pm 0.7 68 34  
2:30 pm 0.4 67 35  
4:00 pm 0.0 64 45  

Table 7: UVB & UV index (UVI), late spring

UVB was measured with Solarmeter 6.2 (µW/cm²) and the UV index (UVI) with Solarmeter 6.5. Meters were pointed straight at the sky. These readings were taken at the very end of April on a warm, sunny day. Temperature and humidity were recorded as well.

Late April - sunny all day with few clouds:

Time UVB UVI Temp °F Hum % Notes
6:08 am 0 0 51 49  
6:15 am 0 0 51 49 sun rise 6:10 am
6:30 am 1 0 51 47  
6:45 am 3 0 53 46  
7:00 am 6 0 55 39  
7:15 am 12 0 57 37  
7:30 am 19 0.1 59 36  
8:00 am 34 0.2 63 33  
8:15 am 38 0.3 64 33  
8:30 am 52 0.5 65 33  
9:00 am 62 0.7 67 32  
9:15 am 121 1.5 70 32  
9:30 am 133 1.7 71 33  
9:45 am 162 2.3 72 33  
10:00 am 181 2.7 73 33  
10:30 am 212 3.6 74 31  
11:00 am 247 4.1 75 31  
11:30 am 278 5.0 76 27  
12:00 noon 295 5.6 81 22  
12:35 pm 330 6.4 85 20 highest UVB = 330
1:00 pm 329 6.5 88 19 highest UVI = 6.5
1:30 pm 299 5.6 91 18  
2:00 pm 304 5.6 89 18  
2:30 pm 274 4.8 86 18  
3:00 pm 225 3.8 83 19  
3:25 pm 201 3.1 81 20  
4:00 pm 167 2.3 80 20  
4:30 pm 122 1.4 78 21  
5:00 pm 93 0.9 77 22  
5:30 pm 49 0.4 75 24  
6:00 pm 33 0.2 73 24  
6:30 pm 15 0.1 72 28  
7:00 pm 5 0 70 30  
7:30 pm 0 0 68 34 sunset 7:40

Table 8: UVB & UV index (UVI), mid summer

UVB was measured with Solarmeter 6.2 (µW/cm²) and the UV index (UVI) with Solarmeter 6.5. Two separate readings were taken each time. Meters were first pointed straight at the sky and then directly at the sun. Readings format in the table is sky / sun.

These readings were taken mid summer in July on a warm, but cloudy and unusually humid (for this area) day. Summer rain is not common here. The clouds were quickly moving, so the readings were changing fast between meter changes. Thus, the UVB and UVI readings were not always taken in perfectly matching conditions.

Temperature and humidity were recorded with a thermometer / hygrometer that was placed in a shady spot under a bush and a tree.

Mid July - partially cloudy day:

Time UVB UVI Temp °F Hum % Notes
6:40 am 4/2 0/0 71 99 sunny
7:00 am 9/5 0.1/0.1 71 99 partly cloudy
7:30 am 20/13 0.2/0.1 73 94 "
8:00 am 37/34 0.4/0.3 74 86 mostly cloudy
8:30 am 73/89 0.8/1.0 78 80 sunny
9:00 am 108/137 1.4/1.7 81 76 "
9:30 am 93/94 1.5/1.4 82 69 partly cloudy
10:00 am 84/89 1.4/1.4 81 72 mostly cloudy
10:30 am 109/113 2.0/2.1 82 58 "
11:00 am 258/260 5.2/5.7 82 56 sunny w. clouds
11:30 am 315/324 6.5/6.7 83 54 "
12:00 noon 345/349 7.3/7.2 86 46 "
12:30 pm 349/341 7.8/7.7 86 47 mostly sunny
1:00 pm 352/347 8.1/7.8 86 49 sun, highest UVI = 8.1
1:30 pm 348/329 7.9/7.4 85 49 mostly sunny
2:00 pm 330/287 7.1/6.4 86 49 sunny
2:30 pm 309/300 6.5/6.3 84 50 "
3:00 pm 276/283 5.5/5.3 85 51 "
3:30 pm 236/231 4.3/4.3 85 51 "
4:00 pm 196/206 3.3/3.5 84 51 "
4:30 pm 107/95 1.6/1.4 85 49 partly cloudy
5:00 pm 84/61 1.2/0.9 86 47 "
5:30 pm 33/24 0.4/0.3 82 54 cloudy
6:00 pm 3/3 0.4/0.3 80 59 dark clouds, no sun
6:30 pm - - - - rain started 6:10 pm

Measuring indoor UVB & UVI

Note: Measure UVB with Solarmeter 6.2 and UVI with Solarmeter 6.5.

a.) UVB

If your tortoise has limited access to outdoor sun, using reptile UVB bulbs from reputable, well known companies is a good way to boost his UVB exposure.

When measuring the UVB emission of an indoor reptile bulb with the Solarmeter 6.2, it's common to hold the meter 12" away from the bulb. This is probably the most common reading distance among hobbyist because it allows for data comparison (identical bulbs only). However, to measure the amount of UVB your tortoise is actually exposed to, you'll need to hold the UV meter sensor at the level of your tortoise's head or limbs where the skin is exposed.

Not all reptile bulbs emit UVB. Reptile bulbs can emit light, heat, UVA, and UVB. Singly or combined. Many reptile basking bulbs emit UVA, but not UVB. Check the box labels.

b.) UVI & Ferguson zones

UV index (UVI) rating with numbers from low 1 to extreme 11+ is used with humans. However, for reptiles, we now have the Ferguson zones! These recommendations are based on UV readings taken of basking reptiles in the wild by Dr Gary Ferguson and his team. Ferguson system consists of four zones, from 1 to 4. The UVI zones 2 and 3 are of special interest to turtle and tortoise keepers and can be used as a guideline to set up indoor UV lighting.

The four Ferguson zones are:

Zone 1

Zone 2

Zone 3

Zone 4

Note: avg = average exposure, max = maximum recorded

Indoor UV lighting for tortoises should provide a UVB gradient (just like we provide a temperature gradient) down to zone 1 level. This allows the tortoise to regulate his UVB exposure. Always provide a shaded area in addition to the UV zone. Blasting reptiles with high levels of UV all day long with no hiding places can cause eye problems, skin damage, and other illnesses.

Source: Zoo Med, Digital UV Index Radiometer instructions, 2013

 

Related page: lighting (reptile UVB bulbs)

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