The Hybrid IR Filter: The Root of the Problem in the Purple Haze Controversy, Pt. 1

by DMR2012 on October 15, 2012

Most small cameras, including those in every generation of iPhone, may exhibit some form of flare at the edge of the frame when capturing an image with out-of-scene light sources. This can happen when a light source is positioned at an angle (usually just outside the field of view) so that it causes a reflection off the surfaces inside the camera module and onto the camera sensor. Moving the camera slightly to change the position at which the bright light is entering the lens, or shielding the lens with your hand, should minimize or eliminate the effect” (Apple’s response to the purple haze controversy. Matt Brian, “Apple posts support advisory to explain and ease iPhone 5 purple lens flare worries”. The Next Web. October 7, 2012; italics mine).

 

In my last post, I briefly discussed the purple haze in photos taken with the iPhone 5 and cited writer Matt Brian’s quote of Apple’s response to the customer complaint about the new phone’s iSight camera. Mashable reported on the purple haze recently and stated that, despite Apple’s claim that the problem arises in sunlight, the purple haze appears in low-light situations as well. The Mashable photo of the iPhone 4 and iPhone 5 cameras can be seen here:

 

The 4S model does not have the purple haze that the iPhone 5 bears. Mashable uses this to argue that the iPhone 5 has a defective iSight camera. The Next Web responds this week to the hype about the iPhone 5 camera defect and provides the following photos to justify its case that the iPhone 5 iSight camera is as normal as any other smartphone camera:

 

Screen Shot 2012 09 26 at 11.57.03 AM 520x112 Apple posts support advisory to explain and ease iPhone 5 purple lens flare worries

[Credit: Andreas Solaro/Getty, provided by The Next Web]

 

While the iPhone 5 is used in both photo demonstrations, the answer can be found in the differences: why is it that Mashable uses the iPhone 4 while The Next Web uses the iPhone 4S? The answer, in a word, is simple: aperture.

 

Phil Schiller thrilled Apple developers and interested media journalists last month as he took to the stage to talk about the iPhone 5 hardware. He told us that the iSight camera was new and improved in the iPhone 5, and came with an f/2.4 aperture. Most photographers understand this language, but many ordinary consumers do not. At this point, I will take time to explain what aperture is and how to assess the aperture of a smartphone or digital camera.

 

When the word “aperture” is used, it refers to either a “large” or “small” number that tells you how much light is being allowed into the smartphone camera. The typical aperture of a smartphone or digital camera starts at f/2.8, but Apple pushed the limits with its f/2.4, an aperture that allows more light into your photos so as to aid them in low-light conditions. If the “f-number” (f/x) increases, the amount of light decreases in your photos, such that everything looks decent but dull. If the “f/x” number decreases, greater light is allowed into your photos for an improved quality overall. Wikipedia provides a great summary of this information:

“The f-stops (f/x.y) that might be found on a typical lens include 2.8, 4, 5.6, 8, 11, 16, 32, where going up ‘one stop’ (using lower f-stop numbers) doubles the amount of light reaching the film, and stopping down one stop halves the amount of light” (http://en.wikipedia.org/wiki/Photography).

 

Imagingresource.com confirms this thought by the following:

“The fixed aperture of the iPhone 4S’ new five-element, fixed-focal-length lens has increased slightly, from f/2.8 in the iPhone 4, to f/2.4 in the iPhone 4S, making it, theoretically, a better performer in low light” (http://www.imaging-resource.com/PRODS/IPHONE4S/IPHONE4SA.HTM)

 

Connected to aperture as a smartphone camera feature is also the idea of shutter speeds. Shutter speeds determine how fast you can take photos while in motion without experiencing shutter lag or blurriness. The fact that the iPhone 5 has an f/2.4 aperture means that your iPhone 5 camera operates 40% faster in its shutter speed than the iPhone 4S. If you are taking pictures of your vacation hotspot as your family drives you to your vacation hotel, you can snap photos without worrying about blurring issues later. If you want to learn more about the relationship between aperture (“f/x.y”) and shutter speeds, read more here.

 

Both the iPhone 4S and iPhone 5 have the same aperture (f/2.4). It makes sense, then, that the iPhone 4S and 5 would share some sort of purple haze on their photos. If aperture pertains to light, and both iPhones allow the same amount of light to pass through to their photos, then both iPhones should contain similar amounts of bright light when photos are taken in sunlight. The aperture makes the two iPhones equal in terms of their light emission, so the aperture cannot be responsible for the iPhone 5’s purple haze. I also researched some information on the iSight camera’s backside illumination sensor (known as a BI), but the sensor cannot be responsible for the iPhone 5’s deep purple haze because it was installed on the iPhone 4S as well as the iPhone 5 (and used for low-light situations). Similar to the aperture, if the backside illumination sensor was at the root of the problem, photos on both the iPhone 4S and iPhone 5 would be brighter in appearance. This still does not account for the purple haze, however.

 

At this point, I was ready in my research to agree with Apple that the angle at which iPhone 5 users hold their iPhones in direct sunlight (and outdoors) accounts for the purple haze photos; this could not be, however, because the same deep, dark hue that the iPhone 5 featured outdoors was the same deep, purple hue that iPhone 5 users experienced indoors. This fact can be seen in the following photos from Warner Crocker of GottaBeMobile. The first photo is taken with the iPhone 4S, and it shows a majority of white light surrounded by a small amount of purple haze under the lamp (but no major purple haze). In the second photo, the iPhone 5 portrait shows a purplish haze by the bedroom lamp, as well as a picture of Crocker and his wife. The purple haze seems to be rather undeniable and unavoidable in the iPhone 5 photo, and one notices that it is much darker in the iPhone 5 than the 4S.

 

Next, I believed that backside illumination could be the culprit; however, the iPhone 4S did not reflect the deep, purple hue of the iPhone 5 picture. While pictures of the iPhone 4S (as evidenced by The Next Web) show purple traces, the purple traces are not as deep or dark as the purple in the iPhone 5 photos. There must be something that the iPhone 5 has (apart from the specifications of the iPhone 4S) that accounts for the deeper purple present in the photos taken with the iPhone 5. This can only be explained by an additional element not native to any other iPhone model. Some sources suggest that the answer can be found in the sapphire crystal lens covering. While this may be the case, we must first find the common denominator between the two iPhones (iPhone 4S and 5) before we can detect the difference that explains the purplish hue of the iPhone 5.

 

What explains the presence of the hue in the iPhone 4S and iPhone 5 (not the purple of the hue)? It would consist of a feature of both iPhone models. The answer to this question can be found in the introduction of the hybrid IR filter for the iPhone 4S, a feature that was manufactured to “fix” the spectral screen display issues in the iPhone 4. Respected tech writer Anand Shimpi writes:

 

“The 4 no doubt has an IR filter (though not a great one), but it’s probably just a thin film rather than a discrete filter right before the sensor. The 4S includes what Apple has deemed a ‘hybrid IR filter’ right on top of the sensor, which is possibly just a combination of UV/IR CUT filter (UV is a problem too), and an anti-aliasing filter… The thin film IR filters that smartphones have used in the past also are largely to blame for some of the color nonuniformity and color spot (magenta/green circle) issues that people have started taking note of. With these thin film IR filters, rays incident on the filter at an angle (as we move across the field) change the frequency response of the filter and the result is that infamous circular color nonuniformity. I wager the other effect is some weird combination of vignetting and the microlens array on the CMOS, but when I saw Apple make note of their improved IR filter my thoughts immediately raced to this ‘hybrid IR filter’ as being their logical cure for the infamous green circle the iPhone 4 exhibits” (http://www.anandtech.com/Show/Index/4971?cPage=4&all=False&sort=0&page=11&slug=apple-iphone-4s-review-att-verizon).

The iPhone 4 infrared (IR) filter is considered to have an infrared filter, which keeps out red light from hitting the camera lens; however, the lack of a UV filter to keep out UV rays may have been responsible for the color splotches reported by numerous iPhone 4 customers. According to The Age, in August 2010, many iPhone 4 users took to numerous forums to express their disgust with the green, blue, and yellow tinges they noticed on their iPhone displays when taking photos. Many photos contained a mixture of the three colors, often having a bluish-green tint with both rear and front-facing cameras in indoor lighting conditions. According to Anand, Apple included a “hybrid IR” filter into its iPhone 4S model, it did so to combat the green tinges and spots that customers saw on the iPhone 4 model. The hybrid IR filter, is composed of two filters (this is why the filter is called a “hybrid”): a UV filter (to block UV rays) and a IR filter (to block the infrared rays). The 4S model, however, proved to be problematic because the newly-added UV filter was not effective: instead of eliminating color altogether, it produced purplish-magenta-white photos, as demonstrated by The Verge photos above.

 

According to Wikipedia, the inferior quality of the iPhone camera filters is responsible for the purple haze present in both the iPhone 4S and iPhone 5:

 

“Cheaper digital cameras and camera phones have less effective filters and can “see” intense near-infrared, appearing as a bright purple-white color. This is especially pronounced when taking pictures of subjects near IR-bright areas (such as near a lamp), where the resulting infrared interference can wash out the image” (http://en.wikipedia.org/wiki/Infrared).

 

The bold phrase in the quote, “appearing as a bright purple-white color,” explains the problem with both the iPhone 4S and iPhone 5 (see Warner Crocker’s photos in an earlier link). Crocker’s photos are taken near a lamp (either out in a main room or the bedroom), the source of the “infrared interference” that causes the purple haze. The purple haze is due, then, to “less effective filters” that have been placed within the iPhone 4S and iPhone 5 cameras. How does the hybrid IR filter cause the purple haze in iPhone photos? That is a subject I will cover in the next post. Stay tuned.

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