Light Runes

Hard light is a valid medium for the Celestial language. As a result, projecting a relevant phrase/concept in Celestial can drastically improve the effectiveness of a projection. The main difficulties come from:
-The various multi-dimensional factors that affect Celestial: the same projected shape will not necessarily have a valid effect if duplicated.
-Difficulty in visualization: Complex runes are simply much more difficult to keep in one's mind than simple geometric shapes. One must be able to execute a rune perfectly in controlled conditions before they can create an implement for it, after all.

Creation

While ordinary arcanism is based around manipulating hard light, it's important to note that hard light is a side effect of the soul manipulations required to create Celestial runes. Hard light is only the 'answer', and one must figure out the question. 2+2 = 4, and 5 - 1 = 4. 4 is like the hard light generated, whereas the actual spell wavelength required is dictated by the left side of the equation: there are infinitely many possible ways to reach the end result. Deriving a light rune is a combination of looking for common threads between past implementations of runes, and huge amounts of trial and error to figure out which wavelengths combine with one's specific soul to generate a given end result. Runes get easier to learn as more archmages publish their specific variations, allowing for more cross-referencing and narrowing of variables. Tear and Conversion are both runes so widely published that the underlying theory for them had been more or less worked out, allowing for the creation of standardized waveforms.

Application

A hard light barrier with the shape roughly correlating to the Celestial rune for "shield" (or block/protect etc.) will be dramatically more efficient at dispersing energy than expected.
Learning a rune is only half the process: figuring out how to shape it for useful applications can take just as much time. A rune intended for simple barriers may be easy enough to translate (notably, this is why learning Reflect is relatively easy- it's a flat, circular plane typically being turned into a shield anyway).

Understanding & Use

It's important to note that while it may take a very long time to make a 'true' version of a rune, one which performs its function correctly and efficiently, one can make use of the principals involved in said rune much earlier in their research. Typically, these will either require far more power to sustain or cause some sort of other inefficiency. As a result, most experienced mages have passing knowledge of several runes and are able to use them in limited applications- true mastery is a daunting task which requires single-minded focus and results in progressively worse diminishing returns on research until it 'clicks' and is instinctively understood. For example, practically every heat mage uses a bastardized variation of Heat's principals to convert their projections' energy to heat more efficiently. The rune itself produces an order of magnitude more, but hacked together ideas based on it are still significantly better than exclusively focusing light into heat through magnifiying lenses etc.

Prodigies & Savants

Extreme natural aptitude for spark manipulation is possible, similarly to inborn talent for any other field. What marks savants as special is that their Celestial Light Rune for a given concept takes an extremely simple shape. For example, being able to write 'persist' with a simple hexagon essentially allows one to bypass several hundred hours of practice. Additionally, one's maximum capacity for magic using the rune is functionally increased, since no amount of training will make one visualize a fractal as easily as a simple geometric shape (which reduces implement overhead costs as well).

Cultural Importance

Formally, one of the requirements for a mage to be recognized as an archmage is to be capable of a Celestial projection. Because no two people write a rune exactly the same in Celestial (again, outside factors), even learning a rune with a very well-documented concept requires extensive training and research. For example, one of the most heavily researched Celestial runes roughly translates to "reflect". While some elitist mages may stigmatize learning it instead of developing one's own Celestial projection, the process still requires several hundred hours of research and training. Successive runes tend to build upon past knowledge and require far less work. Some, like Entropy, essentially require knowledge of other runes (Drain in this case).

What makes a light rune?

So essentially, light runes are spell waveforms which resonate with reality in a way that generates additional effects. The spell waveform's expression needs a hard light component to it, or it wouldn't technically be a light rune.
It's theorized that light runes are created when spell waveforms gain enough Celestial Weight to become artifacts. The exact process behind this is unclear, but Meraxes' Breach is the only example in recent memory. It may be an issue with the era: in the past ages, there were far more gods roaming around to potentially kill and ascend a waveform with. Meraxes had to actively hunt the most dangerous things he could find for centuries before Breach became what it did, and even then it probably has to do with how the last remaining true god on the continent was killed by a heavily-amplified Breach.
Another theory is the idea that light runes are fueled by ancient, esoteric Prophecies & Edicts- part of why they're so complex is to fulfill the edict's activation conditions.
These two theories don't inherently contradict each other- the difference between a conceptual artifact and an edict is just semantics and perspective. It's just more that the two theories popped up in different cultures with little interaction for most of their histories (waveform theory being more of a northern/whirlpool thing, and edicts an abyssian/wastelander/darklit thing- likely reinforced in the latter due to Meraxes' interactions with the aforementioned areas).