Following the era of the Sikh Misls (Sikh confederacies) of the 18th century and leading into the early parts of the 19th century as Maharaja Ranjit Singh consolidated his Empire, we do not find mention of Sikhs wearing helmets, either in early accounts or images.
Their dress is extremely scanty: a pair of long blue drawers, and a kind of checkered plaid, a part of which is fastened round the waist, and ther other thrown over the shoulder, with a mean turban form their clothing and equipage. The chiefs are distinguished by wearing some heavy gold bracelets on their wrists, and sometimes a chain of the same metal bound round their turbans, and by being mounted on better horses: otherwise no distinction appears amongst them. (Forster, 1798) 
Their horsemen use swords and spears, and most of them now carry matchlocks, though some still use the bow and arrow ; a species of arms, for excellence in the use of which their forefathers were celebrated, and which their descendants appear to abandon with great reluctance. (Malcom, 1812) 
This was a time when the Sikh Kingdom of Lahore was fairly isolated from the outside world including that of European explorers and travellers. Although the area of Punjab south of the Sutlej river was open to British travellers, north of the Sutlej river in the lands under Maharaja Ranjit Singh, little was known or had been seen.
The first major shift in this isolation occurred with the meeting between Maharaja Ranjit Singh and the British Governor General of India, Lord William Bentinck on the banks of the Sutlej river at Ropar in October 1831. With this meeting came a gradual shift in Maharaja Ranjit Singhs policy of isolation and the slow but steady procession of travellers to the Lahore Kingdom began in the 1830’s, providing first hand eyewitness accounts of Sikhs wearing helmets, both in the Royal Courts as well as in the army.
W.G. Osborne the Military Secretary to the Governor-General of India Lord Auckland who visited the Sikh Kingdom in May 1838 provided the following detailed description of one of the members of the royal court, Raja Suchet Singh, brother of minister Dhian Singh:
He is high in Runjeet's favour, as well as much respected and admired by all the Sikhs, and about twenty-eight years of age. His dress was magnificent; a helmet or scull cap of bright polished steel inlaid with gold, and a deep fringe of chain mail, of the same material reached to his shoulders, three plumes of black heron's feathers waving on his crest, and three shawls of lilac, white, and scarlet, twisted very round and tight, interlaced with one another and gathered round the edge of the helmet, a chelenk of rubies and diamonds on his forehead. (Osborne, 1840) 
In describing this Indo-Persian style helmet with it’s kalgi plumed apex, Osborne provides important details of how turban material was tied around the edge of the helmet. This same style of helmet with turban material wrapped around it was also depicted in a drawing done by Osborne of Raja Sher Singh.
Baron Charles Hugel who visited Lahore in 1836 provides another description of Rajah Suchet Singh’s helmet:
Kushal Singh, the Jemidar, and Raja Sushet Singh, were the only state officers present. The last, a very fine distinguished man, wore a black and gold enameled helmet with the visor open, and ornamented with three large black herons feathers. (Hugel, 1845) 
The ‘visor open’ in Hugel’s description refers to the Indo-Persian helmets nasal-guard in the raised position.
The member of the Lahore Royal Court most often depicted wearing a helmet is Maharaja Ranjit Singh’s step-son Raja Sher Singh. From early drawings of the Raja by Osborne and Vigne done between 1836 to 1838 we see Sher Singh wearing a Indo-Persian style helmet with turban material wrapped around it’s lower edges. When Sher Singh eventually succeeded to the throne of the Sikh Kingdom in 1841 we still find images of the Maharaja wearing a helmet including the famous drawing by Prince Soltykoff of Maharaja Sher Singh and his entourage as well as a painting done by a native artist showing Maharaja Sher Singh on horseback with a helmet.
In the latter era of the Sikh Kingdom another member of the Royal Court, Raja Lal Singh has also been depicted wearing a helmet. Raja Lal Singh who was appointed Chief Minister in December 1845 and who led Sikh forces against the British during the First Anglo-Sikh War appears in two different drawings wearing French cuirass body armour as previously worn in Maharaja Ranjit Singh’s French General Allard’s Curassiers cavalry regiment and a Indo-Persian style helmet.
Raja Dhian Singh, Maharaja Ranjit Singh’s Prime Minister also wore a unique type of helmet and details of this are covered in the section on Turban Helmets.
1. George Forster, A Journey from Bengal to England, Volume I (London: 1798), 290-291
2. John Malcolm, Sketch of the Sikhs (London: 1812), 141
3. W. G. Osborne, The Court and Camp of Runjeet Sing (London: 1840) 62-63
4. Baron Charles Hugel, Major T. B. Jervis, Translator, Travels in Kashmir and the Panjab (London: 1845), 301