Tuesday, November 23, 2010

Microvision: What Does 15 Lumens Mean to SHOWwx+ Projected Image Brightness?

Here's the news...

MicroVision Unveils Second-Generation Laser Pico Projector, SHOWWX+

Press Release
Source: MicroVision, Inc.
Monday November 22, 2010, 1:00 pm EST

REDMOND, Wash.--(BUSINESS WIRE)-- MicroVision, Inc. (NASDAQ:MVIS - News), the leader in innovative ultra-miniature laser display technology, today unveiled SHOWWX+®, the second-generation of its award-winning laser pico projector. With a 50 percent brighter display within the same slim size, SHOWWX+ enables mobile device users to break free from the small screen and project large, clear 16:9 widescreen content wherever they are, on any surface.

“We’re on a mission to eliminate the squinting and huddling that occurs when mobile device users share content,” said Alexander Tokman, president and CEO of MicroVision. “SHOWWX+ is designed to simply and quickly connect to today’s hottest new portable devices so users can display and share large vibrant images and video with ease.”

Taking the Mobile Experience Beyond the Small Screen
SHOWWX+ is the first laser pico projector to offer a big-screen, movie-length experience that easily slips into a shirt pocket. Boasting two hours of battery life, 15 laser lumens, a contrast ratio up to five times higher than competing products, and the shortest throw ratio of any pico projector on the market, the SHOWWX+ is the perfect accessory for expanding viewing experiences beyond a palm or pad-sized screen. Its category-leading 5,000:1 native contrast ratio ensures the SHOWWX+ produces the deepest blacks and most brilliant whites.


Here’s the link to the news…

In all fairness, this is excellent news as it shows progress in the right direction to achieving a brighter and improved quality image from the tiny laser based PicoP Display Engine at the core of MicroVision’s second generation product SHOWwx+.

We have seen the first generation pico projector SHOWwx… and are really impressed by the 10-lumen brightness and the quality of projected image.

However, with the second generation SHOWwx+ there are lots of speculations and misinformation about what does it really mean to have 15-lumen brightness from the new PDEs?

Consider this in its utmost simplicity…

A lamp [LED or a laser diode] produces a certain amount of light that is measured in lumens. The lumen is the globally standardized SI unit of "luminous flux"--meaning that it measures just how much visible light is produced by an object such as, for example, a light bulb.

Typical indoor lamps have light outputs ranging from 50 to 10,000 lumens. You use lumens to order most types of lamps, to compare lamp outputs, and to calculate lamp energy efficiencies.

The luminous flux is a weighted sum of the power at all wavelengths in the visible band. Light outside the visible band does not contribute. Luminous flux is the total perceived power emitted in all directions. However, luminous intensity is the perceived power per unit solid angle… like as directed and projected on a screen and seen by the eye.

[Note that lumen output is not related to the light distribution pattern of the lamp. A large fraction of a lamp’s lumen output may be useless if it goes in the wrong directions… like as in diffused light from a lamp or LED source]

Luminous intensity:
It is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela (cd).

We are interested in the measurement of visible light as perceived by human eye; the human eye can only see light in the visible spectrum and has different sensitivities to light of different wavelengths within the spectrum. When adapted for bright conditions (photopic vision), the eye is most sensitive to greenish-yellow light at 555 nm. Light with the same radiant intensity at other wavelengths has a lower luminous intensity. For instance, the measured responses of the eye to violet light varied by a factor of ten.

Luminous intensity should not be confused with another photometric unit, luminous flux, which is the total perceived power emitted in all directions. Luminous intensity is the perceived power per unit solid angle.

Lux is the unit that indicates the density of light that falls on a surface. This is what light meters measure. For example, average indoor lighting ranges from 100 to 1,000 lux, and average outdoor sunlight is about 50,000 lux.

One Lux, for example, is measured as one lumen per square meter. The general term for lux is “luminance”.

Lux versus lumen:
The difference between the lux and the lumen is that the lux takes into account the area over which the luminous flux is spread. A flux of 100 lumens, concentrated into an area of one square meter, lights up that square meter with a luminance of 100 lux. However, the same 100 lumens, spread out over ten square meters, produce a dimmer luminance of only 1 lux.

Now consider this…
A person looking at the screen sees different areas of his visual field in terms of levels of brightness, or luminance, measured in candelas [the measure of luminous intensity] per square meter.

With that clarity on the difference between lumen and lux; conversion between the two is simple.

Lux is a measure of how many lumens are present in a given area. It's essentially a measure of "photon density" or "light concentration." A "denser" cloud of photons [like a 10 lumen laser light pixel] means there are more lumens present in a pixel space… producing more brightness and higher lux as perceived by the eye.

A "lighter" cloud of photons [like a 10 lumen LED lighting the one square meter of the screen] means fewer lumens are present in a pixel space… leading to dimmer conditions and lower lux as perceived by the eye.

To achieve a desired lux level in a given space it may be necessary to use brighter bulbs with higher lumen rating or use many light bulbs, each producing a given number of lumens.

Do the Conversion…
Measure the dimensions of the space that you wish to illuminate, and write down how many square meters of surface area it has. In case of SHOWwx with 10 lumens, it is 10 lumens over the size of a pixel. In case of 10 lumen LED based pico projectors, it is 10 lumens over the one square meter of the screen.

The fundamental ratio of conversion from desired lux level to required lumens is:

1 lux = 1 lumen per square meter.

Since perceived brightness corresponds to a logarithmic function, depending on the shape of the function's graph, close to the x axis (where pico projector brightness is now), you would tend to see more difference from smaller amounts of change. That's because close to the axis, the curve is relatively steep. The technical projector literature, the ANSI definitions and viewers personal experience says…

“… In case of pico projectors, as you double the ANSI lumens you double the brightness."

However, as the lumen numbers get bigger─ like 2,000 to 4,000 lumens for lamp based projectors, the curve flattens out, so you need relatively more change in lumens to get the same perceived change in brightness.

"For pico projectors, depending on where you are on that curve, it might even be possible to get a “more” than double brightness increase from a smaller than double lumen increase."

In summary…
Lumen for lumen, the laser pico projectors have higher perceived brightness as compared to those using other light sources… and as you increase the ANSI lumens by 50% [going from 10 lumens to 15 lumens] you more or less increase the brightness by 50%.

Anant Goel