Colorimetry Research CR-250-RH Spectroradiometer
Reference Spectro with big performance in a small size
The CR-250 is a reference spectroradiometer ($6,990 MSRP) from a new company, Colorimetry Research Inc. The all metal design is rugged and practical. What is likely to surprise most people seeing the instrument for the first time is how small it is. Resembling a telescopic sight, it is only 7 inches long, 1.5 inches in diameter, and weighs less than a pound. The CR-250 is almost identical in appearance to the CR-100, the company's very nice professional colorimeter, which is pictured above. The only visible difference is that the CR-250 is slightly longer.
Design
The CR-250 connects to a PC via USB, which also provides the necessary power. The data interface is rs-232, which operates over the USB connection using an internal usb/serial adapter and an in-house virtual COM port driver. It also includes a tripod mount with several mounting slots for placement flexibility. The company also offers a dual mount that allows the user to setup the 100 and 250 side-by-side, which is an ideal arrangement when using the CR-250 to correct the CR-100.
The FOV is quite narrow. For example, when placed one meter from the target the spot size is only 1.8 inches (46 mm) in diameter. Since the instrument has no aiming mechanism this is a useful design choice. Given its small size and narrow FOV, most will find it quite easy to get a good aim point merely by line of sight.
The instrument ships with a nice carrying case with a usb cable and the company's own in-house software. Most will probably opt for custom software, such as ChromaPure which now provides support.
There are two internal operating modes related to refresh rate detection and synchronization. The meter's sync feature can be turned off or entered manually. Unlike the CR-100, no automatic sync detection is provided. Sync is a particularly useful feature and no doubt contributes to the instrument's excellent repeatability, even at very low light levels.
Performance
The performance of a reference spectroradiometer is mostly a function of two measurable criteria.
- Low-light sensitivity
- Speed
The two other typical standards of performance, color accuracy and repeatability, are assumed as a consequence of the instrument's design. A properly calibrated spectro of 5nm or better optical resolution should provide accuracy to within ±0.0015xy and repeatability to within ±0.0005 within a specified luminance range.
I compared the CR-250 to my in-house reference spectro, a JETI 1211, and the instruments seemed to agree to very close tolerances, so I have no doubts about the CR-250's color accuracy. Its repeatability was even better than the JETIs, especially on certain display types.
Speed and Low-light sensitivity
Traditionally, the biggest drawback to a spectroradiometer has been its relatively low sensitivity. They can be quite slow at low light levels and may not read 10% video at all. Currently, the speed champ among reference spectros is the JETI 1211. Comparing the CR-250 to the JETI instrument is not straightforward, because the CR instrument has four different operating modes:
- Slow
- Normal
- Fast
- 2x Fast
To complicate matters further, the CR-250 oddly measures at different speeds depending on the display type. [See Manufacturer's Comment] For example, it can require nearly three times longer (given a fixed operating mode) to measure an LED backlit display compared to the time required to measure a refresh display (plasma/CRT), traditional CCFL LCD, or front projector.
With the CR-250 in Fast mode, this is how the speeds compare in seconds.
% Video |
cd/m2 |
JETI 1211 |
CR-250 LCD |
CR-250 LED |
100% |
118.9 |
0.3 |
1.2 |
5.0 |
90% |
94.6 |
0.4 |
1.5 |
6.2 |
80% |
73.7 |
0.5 |
1.9 |
7.5 |
70% |
55.2 |
0.6 |
2.6 |
7.5 |
60% |
39.7 |
0.9 |
3.6 |
7.5 |
50% |
27.4 |
1.3 |
4.9 |
7.5 |
40% |
17.1 |
2.0 |
7.5 |
7.5 |
30% |
9.6 |
3.7 |
7.5 |
7.5 |
20% |
4.1 |
8.9 |
7.5 |
7.5 |
10% |
0.69 |
44.1 |
7.5 |
7.5 |
When measuring anything but an LED, the CR-250's speed is comparable to the 1211 at the top end. Both are quite fast. However, at about 70% video and below the 1211 gains an appreciable advantage. Note that in the Fast mode the CR-250 never gets any slower than 7.5 seconds. This highlights another unusual feature of this instrument. The different speed settings of the instrument not only affect the time required to measure light sources, but it also establishes a maximum time for any reading, regardless of its luminance. These are fixed values at
- Slow (60 secs.)
- Normal (15 secs.)
- Fast (7.5 secs.)
- 2x Fast (3.75 secs.)
Many spectroradiometers—even those that have different read modes—establish a minimum level of saturation to the sensors before a reading can be successfully taken. The JETI 1211 works this way. The read time depends on how long is required for the required sensor saturation. The Colorimetry Research approach has one drawback here. You can never be certain that the maximum exposure time is sufficient to saturate the sensors to the degree required to get an accurate reading. This is only an issue at the very low end, but it poses a challenge to those who may wish to use this instrument as a primary working color analyzer that must take measurements throughout the entire video range. You may need to switch the read mode to maintain accuracy at low signal levels, and it is not obvious--except for trial and error--when this would be necessary. As a practical matter, the Normal setting will be needed for projectors (and even the Slow mode at the very low end), and you should probably switch to Normal mode for 10% readings on flat panels.
The bottom line that that this instrument—as I would argue with all spectroradiometers—is best used as a reference against which a tristimulus colorimeter (such as the CR-100) is corrected. Regardless of the attempts of engineers to make a spectroradiometer read faster and measure lower, they are always going to be much slower than a good colorimeter.
So, how does the CR-250's speed compare to the JETI 1211? When measuring non-LEDS, for any reading between 70%–30% the JETI easily bests the CR-250. If you are measuring an LED, then the JETI is almost always significantly faster. To complicate matters even further, when measuring at 10%, you really need to use the Normal mode for flat panel displays and Slow for projectors. Otherwise, you lose some accuracy. In fact, projectors benefit somewhat from the Slow setting even at 20%. Thus, the 7.5 second maximum read time above is not realistic for the entire video range. 15 to 60 seconds is a more realistic figure.
Summing up
I was quite impressed with the CR-250. It manages to provide reference performance with good speed and low light sensitivity. It does this at a relatively low price. The JETI spectros are the only reference instruments in the same price range. It's closest competitor, the JETI 1201, has worse repeatability and cannot touch the CR-250's speed or sensitivity. The JETI 1211 offers excellent repeatability and is generally faster—sometimes significantly faster—than the CR-250. However, the JETI 1211 is also about 20% more expensive. The new JETI 1501 closes that price gap with performance similar to the 1211, so it is a more realistic competitor to the CR-250.
I do have a few minor quibbles. I initially found the virtual Com port driver for both the CR-100 and CR-250 to be a little finicky. I have found myself reinstalling the driver a couple of times and/or rebooting the PC when the instrument would fully respond. These measures have always fixed the problem, but they are an annoyance. However, subsequent versions of the driver seem to have solved these problems, and I no longer see any issues.
When it comes to price/performance ratio of the CR-250 I cannot find any reason for criticism. It is a great reference spectro on the market at its price point. It also forms half of a compelling professional combination along with the CR-100 colorimeter. Just a few years ago $6990 for a fast, sensitive reference spectroradiometer would have been considered unthinkable. No longer.
| The observations about different exposures times for different technologies [LED and others] is correct, and this can be explained by the fact that the exposure algorithm adjusts the exposure times relative to peak light level on the sensor, not by the average luminance. This is to prevent pixel saturation/overload.
The green component on the CCFL [LCD] is very sharp compared to the green component on the [LED]. Both will provide the same Luminance level but the shape of the signal is what makes the difference in exposure times. |
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